Semiconductor device and display device including the semiconductor device

ABSTRACT

To reduce parasitic capacitance in a semiconductor device having a transistor including an oxide semiconductor. The transistor includes a first gate electrode, a first gate insulating film over the first gate electrode, an oxide semiconductor film over the first gate insulating film, and source and drain electrodes electrically connected to the oxide semiconductor film. The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side and a second oxide semiconductor film over the first oxide semiconductor film. The atomic proportion of In is larger than the atomic proportion of M (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf) in the first oxide semiconductor film, and the atomic proportion of In in the second oxide semiconductor film is smaller than that in the first oxide semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductor device including an oxide semiconductor film and a display device including the semiconductor device.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, or a driving method thereof.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each an embodiment of a semiconductor device. An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic device may each include a semiconductor device.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.

A reliable semiconductor device in which stable electrical characteristics are applied to a transistor using an oxide semiconductor is disclosed (e.g., see Patent Document 1). In the semiconductor device, oxide semiconductor films with different compositions are stacked so that an oxide semiconductor film containing many In is provided on the channel side and an oxide semiconductor film containing many stabilizers such as Ga is provided on the back-channel side.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. 2013-175715

SUMMARY OF THE INVENTION

As a transistor including an oxide semiconductor film as an active layer, a transistor (an L_(ov) transistor or the like in this specification) in which a gate electrode and a source electrode partly overlap with each other and the gate electrode and a drain electrode partly overlap with each other is often manufactured. Meanwhile, as for a transistor (an L_(off) transistor or the like in this specification) in which a gate electrode does not overlap with a source electrode or a drain electrode, it is difficult to reduce the electrical resistance of a region of an oxide semiconductor film where the gate electrode does not overlap with the source electrode or the drain electrode; as a result, the field-effect mobility of the transistor is decreased.

However, in the case where an L_(ov) transistor is used as a pixel transistor of a liquid crystal display, parasitic capacitance is generated between a gate electrode and a source electrode and between the gate electrode and a drain electrode. A time constant in operation becomes large owing to the parasitic capacitance, which makes the rise time and the fall time of a pulse voltage long. Thus, in driving of a display portion, it is difficult to increase a voltage applied to a liquid crystal element or a capacitor to a necessary value. According to an increase in a panel size of a display device such as a liquid crystal display or an organic EL display or an increase in definition of a display screen including a pixel, the above problem becomes serious; therefore, it is difficult to manufacture a panel with high definition and a large screen size using an L_(ov) transistor with high parasitic capacitance.

It is effective to provide a circuit for correcting variation in transistor characteristics or degradation of an organic EL element in a pixel of an organic EL display; however, large parasitic capacitance requires a complicated correction circuit.

In view of the above problems, an object of one embodiment of the present invention is to provide a semiconductor device with low parasitic capacitance. Another object of one embodiment of the present invention is to provide a high-definition display device with a large panel size. Another object of one embodiment of the present invention is to provide a display device with high display quality. Another object of one embodiment of the present invention is to provide a novel semiconductor device. Another object of one embodiment of the present invention is to provide a novel display device.

Note that the description of the above object does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Objects other than the above objects will be apparent from and can be derived from the description of the specification and the like.

One embodiment of the present invention is a semiconductor device including a transistor. The transistor includes a first gate electrode, a first gate insulating film over the first gate electrode, an oxide semiconductor film over the first gate insulating film, a source electrode electrically connected to the oxide semiconductor film, and a drain electrode electrically connected to the oxide semiconductor film.

The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side and a second oxide semiconductor film over the first oxide semiconductor film. In the first oxide semiconductor film, an atomic proportion of In is larger than an atomic proportion of M (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf). The second oxide semiconductor film has a smaller atomic proportion of In than the first oxide semiconductor film.

The oxide semiconductor film includes a first region overlapping with the first gate electrode, a second region overlapping with the source electrode, and a third region overlapping with the drain electrode. The first region does not include the second region and the third region.

Another embodiment of the present invention is a semiconductor device including a transistor. The transistor includes a first gate electrode, a first gate insulating film over the first gate electrode, an oxide semiconductor film over the first gate insulating film, a source electrode electrically connected to the oxide semiconductor film, a drain electrode electrically connected to the oxide semiconductor film, a second gate insulating film over the oxide semiconductor film, and a second gate electrode over the second gate insulating film.

The oxide semiconductor film includes a first region overlapping with the first gate electrode, a fourth region overlapping with the second gate electrode, a second region overlapping with the source electrode, and a third region overlapping with the drain electrode. The first region does not include the second region and the third region. The fourth region does not include the second region and the third region.

The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side and a second oxide semiconductor film over the first oxide semiconductor film. In the first oxide semiconductor film, an atomic proportion of In is larger than an atomic proportion of M (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf). The second oxide semiconductor film has a smaller atomic proportion of In than the first oxide semiconductor film.

The oxide semiconductor film includes a first region overlapping with the first gate electrode, a fourth region overlapping with the second gate electrode, a second region overlapping with the source electrode, and a third region overlapping with the drain electrode. The first region or the fourth region does not include the second region and the third region.

Another embodiment of the present invention is a semiconductor device including a transistor. The transistor includes an oxide semiconductor film, a source electrode electrically connected to the oxide semiconductor film, a drain electrode electrically connected to the oxide semiconductor film, a first gate insulating film over the oxide semiconductor film, and a first gate electrode over the first gate insulating film.

The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side and a second oxide semiconductor film over the first oxide semiconductor film. In the first oxide semiconductor film, an atomic proportion of In is larger than an atomic proportion of M (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf). The second oxide semiconductor film has a smaller atomic proportion of In than the first oxide semiconductor film.

The oxide semiconductor film includes a first region overlapping with the first gate electrode, a second region overlapping with the source electrode, and a third region overlapping with the drain electrode. The first region does not include the second region and the third region.

Another embodiment of the present invention is a semiconductor device including a transistor. The transistor includes a first gate electrode, a first gate insulating film over the first gate electrode, an oxide semiconductor film over the first gate insulating film, a source electrode electrically connected to the oxide semiconductor film, and a drain electrode electrically connected to the oxide semiconductor film.

The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side and a second oxide semiconductor film over the first oxide semiconductor film. In the first oxide semiconductor film, an atomic proportion of In is larger than an atomic proportion of M (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf). The second oxide semiconductor film has a smaller atomic proportion of In than the first oxide semiconductor film.

The oxide semiconductor film includes a first region overlapping with the first gate electrode, a second region overlapping with the source electrode, and a third region overlapping with the drain electrode. The first region does not include the second region or the third region.

Another embodiment of the present invention is a semiconductor device including a transistor. The transistor includes a first gate electrode, a first gate insulating film over the first gate electrode, an oxide semiconductor film over the first gate insulating film, a source electrode electrically connected to the oxide semiconductor film, a drain electrode electrically connected to the oxide semiconductor film, a second gate insulating film over the oxide semiconductor film, and a second gate electrode over the second gate insulating film.

The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side and a second oxide semiconductor film over the first oxide semiconductor film. In the first oxide semiconductor film, an atomic proportion of In is larger than an atomic proportion of M (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf). The second oxide semiconductor film has a smaller atomic proportion of In than the first oxide semiconductor film.

The oxide semiconductor film includes a first region overlapping with the first gate electrode, a fourth region overlapping with the second gate electrode, a second region overlapping with the source electrode, and a third region overlapping with the drain electrode. The first region or the fourth region does not include the second region or the third region.

Another embodiment of the present invention is a semiconductor device including a transistor. The transistor includes an oxide semiconductor film, a source electrode electrically connected to the oxide semiconductor film, a drain electrode electrically connected to the oxide semiconductor film, a first gate insulating film over the oxide semiconductor film, and a first gate electrode over the first gate insulating film.

The oxide semiconductor film includes a first oxide semiconductor film on the first gate electrode side and a second oxide semiconductor film over the first oxide semiconductor film. In the first oxide semiconductor film, an atomic proportion of In is larger than an atomic proportion of M (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf). The second oxide semiconductor film has a smaller atomic proportion of In than the first oxide semiconductor film.

The oxide semiconductor film includes a first region overlapping with the first gate electrode, a second region overlapping with the source electrode, and a third region overlapping with the drain electrode. The first region does not include the second region or the third region.

In each of the above structures, the semiconductor device includes a scan line and a data line. The first gate electrode is electrically connected to the scan line, and the source electrode is electrically connected to the data line. An insulating film having a larger thickness than the first gate insulating film is preferably provided between the scan line and the data line in a region where the scan line and the data line intersect with each other.

In any of the above structures, it is preferable that the oxide semiconductor film contain In, M, and Zn, and M is preferably Ga. In any of the above structures, it is preferable that the oxide semiconductor film include a crystal part, the crystal part include a portion, and the c-axis of the portion be parallel to a normal vector of a surface where the oxide semiconductor film is formed.

In any of the above structures, it is preferable that the first region include a portion in which the proportion of the crystal part is larger than the second region. In any of the above structures, it is preferable that the first region include a portion with lower concentration of hydrogen than the second region.

Another embodiment of the present invention is a display device including the semiconductor device according to any of the above structures and a display element. Another embodiment of the present invention is a display module including the display device and a touch sensor. Another embodiment of the present invention is an electronic device including the semiconductor device according to any of the above structures, the display device, or the display module, and an operation key or a battery.

According to one embodiment of the present invention, a semiconductor device with small parasitic capacitance can be provided. According to one embodiment of the present invention, a high-definition display device with a large panel size can be provided. According to one embodiment of the present invention, a display device with high display quality can be provided. According to one embodiment of the present invention, a novel semiconductor device can be provided. According to one embodiment of the present invention, a novel display device can be provided.

Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIG. 2 is a cross-sectional view illustrating one embodiment of a semiconductor device.

FIGS. 3A to 3C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 4A to 4D are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 5A to 5D are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 6A to 6C are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 7A to 7H are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 8A to 8C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 9A to 9C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 10A to 10C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 11A-1 to 11B-2 are top views and cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 12A-1 to 12B-2 are top views and cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 13A-1 to 13B-2 are top views and cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 14A-1 to 14B-2 are top views and cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 15A-1 to 15B-2 are top views and cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 16A-1 to 16B-2 are top views and cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 17A-1 to 17B-2 are top views and cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 18A-1 to 18B-2 are top views and cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 19A to 19D are Cs-corrected high-resolution TEM images of a cross section of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS.

FIGS. 20A to 20D are Cs-corrected high-resolution TEM images of a plane of a CAAC-OS.

FIGS. 21A to 21C show structural analysis of a CAAC-OS and a single crystal oxide semiconductor by XRD.

FIGS. 22A and 22B show electron diffraction patterns of a CAAC-OS.

FIG. 23 shows a change of crystal parts of an In—Ga—Zn oxide owing to electron irradiation.

FIGS. 24A and 24B are schematic diagrams illustrating deposition models of a CAAC-OS and an nc-OS.

FIGS. 25A to 25C illustrate an InGaZnO₄ crystal and a pellet.

FIGS. 26A to 26D are schematic diagrams illustrating a deposition model of a CAAC-OS.

FIG. 27 is a top view illustrating one embodiment of a display device.

FIG. 28 is a cross-sectional view illustrating one embodiment of a display device.

FIG. 29 is a cross-sectional view illustrating one embodiment of a display device.

FIGS. 30A to 30C are a block diagram and circuit diagrams illustrating a display device.

FIGS. 31A and 31B are a top view and a cross-sectional view illustrating one embodiment of a semiconductor device.

FIG. 32A is a block diagram of a display portion, FIG. 32B is an equivalent circuit diagram of a pixel circuit included in a pixel of the display portion, and FIG. 32C is a schematic view of polarized light transmittance versus voltage characteristics of a liquid crystal element.

FIGS. 33A to 33D are timing charts and a schematic view showing operations of a pixel circuit of a pixel portion in a display device.

FIG. 34 illustrates a display module.

FIGS. 35A to 35G illustrate electronic devices.

FIGS. 36A to 36G illustrate electronic devices.

FIGS. 37A and 37B are top views illustrating structures of semiconductor devices in Example.

FIGS. 38A and 38B show Id-Vg characteristics of transistors according to Example.

FIGS. 39A-1 to 39C are a top view and cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 40A to 40C are a cross-sectional view and circuit diagrams illustrating embodiments of a semiconductor device.

FIG. 41 shows a structural example of an RF device tag of one embodiment.

FIG. 42 shows a structural example of a CPU of one embodiment.

FIG. 43 is a circuit diagram of a memory element of one embodiment.

FIGS. 44A and 44B are top views each illustrating a device of one embodiment of the present invention.

FIGS. 45A and 45B are block diagrams each illustrating a device of one embodiment of the present invention.

FIGS. 46A and 46B are cross-sectional views each illustrating a device of one embodiment of the present invention.

FIG. 47 is a cross-sectional view illustrating a device of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings. The embodiments can be implemented with various modes. It will be readily appreciated by those skilled in the art that modes and details can be changed in various ways without departing from the spirit and scope of the present invention. Thus, the present invention should not be interpreted as being limited to the following description of the embodiments.

Note that in this specification, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not limit the components numerically.

In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. In addition, the transistor has a channel region between a drain (a drain terminal, a drain region, or a drain electrode) and a source (a source terminal, a source region, or a source electrode), and current can flow through the drain, the channel region, and the source. Note that in this specification and the like, a channel region refers to a region through which current mainly flows.

Functions of a source and a drain might be switched when transistors having different polarities are employed or a direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be switched in this specification and the like.

Note that in this specification and the like, the expression “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring.

Note that in this specification and the like, a “silicon oxynitride film” refers to a film that includes oxygen at a higher proportion than nitrogen, and a “silicon nitride oxide film” refers to a film that includes nitrogen at a higher proportion than oxygen.

In describing structures of the invention with reference to the drawings in this specification and the like, common reference numerals are used for the same portions in different drawings.

In this specification and the like, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and thus includes the angle greater than or equal to −5° and less than or equal to 5°. The term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and thus includes the angle greater than or equal to 85° and less than or equal to 95°.

In this specification and the like, the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. The term “insulating film” can be changed into the term “insulating layer” in some cases.

Embodiment 1

In this embodiment, a semiconductor device that is one embodiment of the present invention and a method of manufacturing the semiconductor device are described with reference to FIGS. 1A to 1C, FIG. 2, FIGS. 3A to 3C, FIGS. 4A to 4D, FIGS. 5A to 5D, FIGS. 6A to 6C, FIGS. 7A to 7H, FIGS. 10A to 10C, FIGS. 11A-1, 11A-2, 11B-1, and 11B-2, FIGS. 12A-1, 12A-2, 12B-1, and 12B-2, FIGS. 13A-1, 13A-2, 13B-1, and 13B-2, FIGS. 14A-1, 14A-2, 14B-1, and 14B-2, FIGS. 15A-1, 15A-2, 15B-1, and 15B-2, FIGS. 16A-1, 16A-2, 16B-1, and 16B-2, FIGS. 17A-1, 17A-2, 17B-1, and 17B-2, and FIGS. 18A-1, 18A-2, 18B-1, and 18B-2.

<Structure Example 1 of Semiconductor Device>

FIG. 1A is a top view of a transistor 100 that is a semiconductor device of one embodiment of the present invention. FIG. 1B is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 1A. FIG. 1C is a cross-sectional view taken along a dashed-dotted line Y1-Y2 in FIG. 1A. Note that in FIG. 1A, some components of the transistor 100 (e.g., an insulating film serving as a gate insulating film) are not illustrated to avoid complexity. The direction of the dashed-dotted line X1-X1 may be called a channel length direction, and the direction of the dashed-dotted line Y1-Y2 may be called a channel width direction. As in FIG. 1A, some components are not illustrated in some cases in top views of transistors described below.

The transistor 100 includes a conductive film 104 functioning as a first gate electrode over a substrate 102, an insulating film 106 over the substrate 102 and the conductive film 104, an insulating film 107 over the insulating film 106, an oxide semiconductor film 108 over the insulating film 107, a conductive film 112 a functioning as a source electrode electrically connected to the oxide semiconductor film 108, and a conductive film 112 b functioning as a drain electrode electrically connected to the oxide semiconductor film 108. Over the transistor 100, specifically, over the conductive films 112 a and 112 b and the oxide semiconductor film 108, insulating films 114, 116, and 118 are provided. The insulating films 114, 116, and 118 function as protective insulating films for the transistor 100.

The conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 112 b functioning as the drain electrode. In this specification, a transistor in which a gate electrode does not overlap with a source electrode or a drain electrode is referred to as an L_(off) transistor or the like. In this specification, a transistor having a structure in which at least one of a plurality of gate electrodes includes a region not overlapping with a source electrode or a drain electrode is referred to as an L_(off) transistor or the like. Furthermore, in this specification, a transistor including a region where all of gate electrodes and a source electrode overlap with each other and a region where all of the gate electrodes and a drain electrode overlap with each other is referred to as an L_(ov) transistor or the like.

The oxide semiconductor film 108 includes a first region overlapping with the conductive film 104, a second region overlapping with the conductive film 112 a, and a third region overlapping with the conductive film 112 b. The first region is provided not to include the second region and the third region. In other words, the first region is provided not to overlap with the second region and the third region.

The oxide semiconductor film 108 includes a region not overlapping with the conductive film 104, the conductive film 112 a, and the conductive film 112 b. The region where the oxide semiconductor film 108 does not overlap with the conductive film 104, the conductive film 112 a, and the conductive film 112 b corresponds to a region 122.

With the L_(off) transistor, parasitic capacitance between the first gate electrode and the source electrode and between the first gate electrode and the drain electrode can be reduced. The use of a transistor with small parasitic capacitance for a display panel can make a time constant small; thus, a high-definition display device with a large panel size can be provided. Alternatively, the use of a transistor with small parasitic capacitance for a liquid crystal display can suppress feedthrough or the like; thus, a display device with high display quality can be provided.

The oxide semiconductor film 108 includes a first oxide semiconductor film 108 a on the conductive film 104 side and a second oxide semiconductor film 108 b over the first oxide semiconductor film 108 a. The conductive film 104 serves as the first gate electrode. The insulating films 106 and 107 function as first gate insulating films of the transistor 100.

An In-M oxide (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf) or an In-M-Zn oxide can be used for the oxide semiconductor film 108. It is particularly preferable to use an In-M-Zn oxide for the oxide semiconductor film 108.

The first oxide semiconductor film 108 a includes a first region in which the atomic proportion of In is larger than the atomic proportion of M. The second oxide semiconductor film 108 b includes a second region in which the atomic proportion of In is smaller than that in the first oxide semiconductor film 108 a. The second region includes a portion thinner than the first region.

The first oxide semiconductor film 108 a including the first region in which the atomic proportion of In is larger than that of M can increase the field-effect mobility (also simply referred to as mobility or μFE) of the transistor 100.

In the L_(off) transistor, a current flowing when the transistor is turned on (on-state current) tends to be decreased as compared with the L_(ov) transistor because series resistance occurs in the region of the oxide semiconductor film where the first gate electrode does not overlap with the source or the drain. However, when the atomic proportion of In is larger than the atomic proportion of M in the first oxide semiconductor film 108 a, resistance of the region of the oxide semiconductor film where the first gate electrode does not overlap with the source or the drain can be reduced. Consequently, a reduction in on-state current can be suppressed. Thus, an L_(off) transistor can be applied to a variety of semiconductor devices and display devices.

For example, the use of the transistor with high field-effect mobility for a gate driver that generates a gate signal (specifically, a demultiplexer connected to an output terminal of a shift register included in a gate driver) allows a semiconductor device or a display device to have a narrow frame.

On the other hand, the first oxide semiconductor film 108 a including the first region in which the atomic proportion of In is larger than that of M makes it easier to change electrical characteristics of the transistor 100 in light irradiation. However, in the semiconductor device of one embodiment of the present invention, the second oxide semiconductor film 108 b is formed over the first oxide semiconductor film 108 a. In addition, the thickness of a portion including a channel region and the vicinity of the channel region in the second oxide semiconductor film 108 b is smaller than the thickness of the first oxide semiconductor film 108 a.

Furthermore, the second oxide semiconductor film 108 b includes the second region in which the atomic proportion of In is smaller than that in the first oxide semiconductor film 108 a and thus has larger Eg than that of the first oxide semiconductor film 108 a. For this reason, the oxide semiconductor film 108 which is a layered structure of the first oxide semiconductor film 108 a and the second oxide semiconductor film 108 b has high resistance to a negative bias stress test with light irradiation.

The amount of light absorbed by the oxide semiconductor film 108 can be reduced during light irradiation. As a result, the change in electrical characteristics of the transistor 100 due to light irradiation can be reduced. In the semiconductor device of one embodiment of the present invention, the insulating film 114 or the insulating film 116 includes excess oxygen. This structure can further reduce the change in electrical characteristics of the transistor 100 due to light irradiation.

Here, the oxide semiconductor film 108 is described in detail with reference to FIG. 2.

FIG. 2 is a cross-sectional enlarged view of the oxide semiconductor film 108 and the vicinity thereof in the transistor 100 shown in FIG. 1B.

In FIG. 2, t1, t2-1, and t2-2 denote a thickness of the first oxide semiconductor film 108 a, one thickness of the second oxide semiconductor film 108 b, and the other thickness of the second oxide semiconductor film 108 b, respectively. The second oxide semiconductor film 108 b over the first oxide semiconductor film 108 a prevents the first oxide semiconductor film 108 a from being exposed to an etching gas, an etchant, or the like when the conductive films 112 a and 112 b are formed. This is why the first oxide semiconductor film 108 a is not or is hardly reduced in thickness. In contrast, in the second oxide semiconductor film 108 b, a portion not overlapping with the conductive films 112 a and 112 b is etched in formation of the conductive films 112 a and 112 b, so that a depression is formed. In other words, a thickness of the second oxide semiconductor film 108 b in a region overlapping with the conductive films 112 a and 112 b is t2-1, and a thickness of the second oxide semiconductor film 108 b in a region not overlapping with the conductive films 112 a and 112 b is t2-2.

As for the relationships between the thicknesses of the first oxide semiconductor film 108 a and the second oxide semiconductor film 108 b, t2-1>t1>t2-2 is preferable. A transistor with the thickness relationships can have high field-effect mobility and less variation in threshold voltage in light irradiation.

When oxygen vacancy is formed in the oxide semiconductor film 108 included in the transistor 100, electrons serving as carriers are generated; as a result, the transistor 100 tends to be normally-on. Therefore, for stable transistor characteristics, it is important to reduce oxygen vacancy in the oxide semiconductor film 108 particularly oxygen vacancy in the first oxide semiconductor film 108 a. In the structure of the transistor of one embodiment of the present invention, excess oxygen is introduced into an insulating film over the oxide semiconductor film 108, here, the insulating film 114 and/or the insulating film 116 over the oxide semiconductor film 108, whereby oxygen is moved from the insulating film 114 and/or the insulating film 116 to the oxide semiconductor film 108 to fill oxygen vacancy in the oxide semiconductor film 108 particularly in the first oxide semiconductor film 108 a.

It is preferable that the insulating films 114 and 116 each include a region (oxygen excess region) including oxygen in excess of that in the stoichiometric composition. In other words, the insulating films 114 and 116 are insulating films capable of releasing oxygen. Note that the oxygen excess region is formed in the insulating films 114 and 116 in such a manner that oxygen is introduced into the insulating films 114 and 116 after the deposition, for example. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be employed.

In order to fill oxygen vacancy in the first oxide semiconductor film 108 a, the thickness of the portion including the channel region and the vicinity of the channel region in the second oxide semiconductor film 108 b is preferably small, and t2-2<t1 is preferably satisfied. For example, the thickness of the portion including the channel region and the vicinity of the channel region in the second oxide semiconductor film 108 b is preferably more than or equal to 1 nm and less than or equal to 20 nm, more preferably more than or equal to 3 nm and less than or equal to 10 nm.

Other constituent elements of the semiconductor device of this embodiment are described below in detail.

<Substrate>

There is no particular limitation on the property of a material and the like of the substrate 102 as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like may be used as the substrate 102. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate 102. Still alternatively, any of these substrates provided with a semiconductor element may be used as the substrate 102. In the case where a glass substrate is used as the substrate 102, a glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured.

Alternatively, a flexible substrate may be used as the substrate 102, and the transistor 100 may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate 102 and the transistor 100. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate 102 and transferred onto another substrate. In such a case, the transistor 100 can be transferred to a substrate having low heat resistance or a flexible substrate as well.

<Conductive Films Functioning as First Gate Electrode and Source and Drain Electrodes>

The conductive film 104 functioning as a first gate electrode and the conductive films 112 a and 112 b functioning as a source electrode and a drain electrode, respectively, can each be formed using a metal element selected from chromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloy including any of these metal element as its component; an alloy including a combination of any of these metal elements; or the like.

Furthermore, the conductive films 104, 112 a, and 112 b may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, an alloy film or a nitride film in which aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium are combined may be used.

The conductive films 104, 112 a, and 112 b can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A Cu—X alloy film (Xis Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be used for the conductive films 104, 112 a, and 112 b. Use of a Cu—X alloy film enables the manufacturing cost to be reduced because wet etching process can be used in the processing.

<Insulating Film Functioning as First Gate Insulating Film>

As each of the insulating films 106 and 107 functioning as first gate insulating films of the transistor 100, an insulating layer including at least one of the following films formed by a plasma enhanced chemical vapor deposition (PECVD) method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. Note that instead of a stacked-layer structure of the insulating films 106 and 107, an insulating film of a single layer formed using a material selected from the above or an insulating film of three or more layers may be used.

The insulating film 106 has a function as a blocking film which inhibits penetration of oxygen. For example, in the case where excess oxygen is supplied to the insulating film 107, the insulating film 114, the insulating film 116, and/or the oxide semiconductor film 108, the insulating film 106 can inhibit penetration of oxygen.

Note that the insulating film 107 that is in contact with the oxide semiconductor film 108 functioning as a channel region of the transistor 100 is preferably an oxide insulating film and preferably includes a region including oxygen in excess of the stoichiometric composition (oxygen-excess region). In other words, the insulating film 107 is an insulating film capable of releasing oxygen. In order to provide the oxygen excess region in the insulating film 107, the insulating film 107 is formed in an oxygen atmosphere, for example. Alternatively, the oxygen excess region may be formed by introduction of oxygen into the insulating film 107 after the deposition. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be employed.

In the case where hafnium oxide is used for the insulating film 107, the following effect is attained. Hafnium oxide has a higher dielectric constant than silicon oxide and silicon oxynitride. Therefore, by using hafnium oxide, the thickness of the insulating film 107 can be made large as compared with the case where silicon oxide is used; thus, leakage current due to tunnel current can be low. That is, it is possible to provide a transistor with a low off-state current. Moreover, hafnium oxide with a crystalline structure has higher dielectric constant than hafnium oxide with an amorphous structure. Therefore, it is preferable to use hafnium oxide with a crystalline structure in order to provide a transistor with a low off-state current. Examples of the crystalline structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited thereto.

In this embodiment, a silicon nitride film is formed as the insulating film 106, and a silicon oxide film is formed as the insulating film 107. The silicon nitride film has a higher dielectric constant than a silicon oxide film and needs a larger thickness for capacitance equivalent to that of the silicon oxide film. Thus, when the silicon nitride film is included in the first gate insulating film of the transistor 100, the physical thickness of the insulating film can be increased. This makes it possible to reduce a decrease in withstand voltage of the transistor 100 and furthermore to increase the withstand voltage, thereby reducing electrostatic discharge damage to the transistor 100.

<Oxide Semiconductor Film>

The oxide semiconductor film 108 can be formed using the materials described above. In the case where the oxide semiconductor film 108 includes In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming the In-M-Zn oxide satisfy In≧M and Zn≧M As the atomic ratio of metal elements of such a sputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:3, In:M:Zn=3:1:2, and In:M:Zn=4:2:4.1 are preferable. In the case where the oxide semiconductor film 108 is formed of In-M-Zn oxide, it is preferable to use a target including polycrystalline In-M-Zn oxide as the sputtering target. The use of the target including polycrystalline In-M-Zn oxide facilitates formation of the oxide semiconductor film 108 having crystallinity. Note that the atomic ratios of metal elements in the formed oxide semiconductor film 108 vary from the above atomic ratio of metal elements of the sputtering target within a range of ±40% as an error. For example, when a sputtering target with an atomic ratio of In to Ga and Zn of 4:2:4.1 is used, the atomic ratio of In to Ga and Zn in the oxide semiconductor film 108 may be approximately 4:2:3.

The first oxide semiconductor film 108 a can be formed using the sputtering target having an atomic ratio of In:M:Zn=2:1:3, In:M:Zn=3:1:2, or In:M:Zn=4:2:4.1. The second oxide semiconductor film 108 b can be formed using the sputtering target having an atomic ratio of In:M:Zn=1:1:1 or In:M:Zn=1:1:1.2. Note that the atomic ratio of metal elements in a sputtering target used for forming the second oxide semiconductor film 108 b does not necessarily satisfy In≧M and Zn≧M, and may satisfy In≧M and Zn<M, such as In:M:Zn=3:2:1.

The energy gap of the oxide semiconductor film 108 is 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more. The use of an oxide semiconductor having a wide energy gap can reduce the off-state current of the transistor 100. In particular, an oxide semiconductor film having an energy gap more than or equal to 2 eV, preferably more than or equal to 2 eV and less than or equal to 3.0 eV is preferably used as the first oxide semiconductor film 108 a, and an oxide semiconductor film having an energy gap more than or equal to 2.5 eV and less than or equal to 3.5 eV is preferably used as the second oxide semiconductor film 108 b. Furthermore, the second oxide semiconductor film 108 b preferably has a higher energy gap than the first oxide semiconductor film 108 a.

Each thickness of the first oxide semiconductor film 108 a and the second oxide semiconductor film 108 b is more than or equal to 3 nm and less than or equal to 200 nm, preferably more than or equal to 3 nm and less than or equal to 100 nm, more preferably more than or equal to 3 nm and less than or equal to 50 nm. Note that the above-described thickness relationship between them is preferably satisfied.

An oxide semiconductor film with low carrier density is used as the second oxide semiconductor film 108 b. For example, the carrier density of the second oxide semiconductor film 108 b is lower than or equal to 1×10¹⁷/cm³, preferably lower than or equal to 1×10¹⁵/cm³, further preferably lower than or equal to 1×10¹³/cm³, still further preferably lower than or equal to 1×10¹¹/cm³.

Note that, without limitation to the compositions and materials described above, a material with an appropriate atomic ratio may be used depending on required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of a transistor. Furthermore, in order to obtain required semiconductor characteristics of a transistor, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of the first oxide semiconductor film 108 a and the second oxide semiconductor film 108 b be set to be appropriate.

Note that it is preferable to use, as the first oxide semiconductor film 108 a and the second oxide semiconductor film 108 b, an oxide semiconductor film in which the impurity concentration is low and the density of defect states is low, in which case the transistor can have more excellent electrical characteristics. Here, the state in which the impurity concentration is low and the density of defect states is low (the amount of oxygen vacancy is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor in which a channel region is formed in the oxide semiconductor film rarely has a negative threshold voltage (is rarely normally on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and thus has a low density of trap states in some cases. Furthermore, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width W of 1×10⁶ μm and a channel length L of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, that is, less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V.

Accordingly, the transistor in which the channel region is formed in the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film can have a small change in electrical characteristics and high reliability. Charges trapped by the trap states in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor whose channel region is formed in the oxide semiconductor film having a high density of trap states has unstable electrical characteristics in some cases. As examples of the impurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, and the like are given.

Hydrogen included in the oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and also causes oxygen vacancy in a lattice from which oxygen is released (or a portion from which oxygen is released). Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor film which contains hydrogen is likely to be normally on. Accordingly, it is preferable that hydrogen be reduced as much as possible in the oxide semiconductor film 108. Specifically, in the oxide semiconductor film 108, the concentration of hydrogen which is measured by SIMS is lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³, further preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, further preferably lower than or equal to 5×10¹⁷ atoms/cm³, and further preferably lower than or equal to 1×10¹⁶ atoms/cm³.

The first oxide semiconductor film 108 a preferably includes a region in which hydrogen concentration is smaller than that in the second oxide semiconductor film 108 b. A semiconductor device including the first oxide semiconductor film 108 a having the region in which hydrogen concentration is smaller than that in the second oxide semiconductor film 108 b can be increased in reliability.

When silicon or carbon that is one of elements belonging to Group 14 is included in the first oxide semiconductor film 108 a, oxygen vacancy is increased in the first oxide semiconductor film 108 a, and the first oxide semiconductor film 108 a becomes an n-type film. Thus, the concentration of silicon or carbon (the concentration is measured by SIMS) in the first oxide semiconductor film 108 a or the concentration of silicon or carbon (the concentration is measured by SIMS) in the vicinity of an interface with the first oxide semiconductor film 108 a is set to be lower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

In addition, the concentration of alkali metal or alkaline earth metal of the first oxide semiconductor film 108 a, which is measured by SIMS, is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal of the first oxide semiconductor film 108 a.

Furthermore, when including nitrogen, the first oxide semiconductor film 108 a easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor including an oxide semiconductor film which contains nitrogen is likely to have normally-on characteristics. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the concentration of nitrogen which is measured by SIMS is preferably set to be, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

The first oxide semiconductor film 108 a and the second oxide semiconductor film 108 b may have a non-single-crystal structure, for example. The non-single crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure, or an amorphous structure, for example. Among the non-single crystal structure, the amorphous structure has the highest density of defect states, whereas CAAC-OS has the lowest density of defect states.

<Insulating Films Functioning as Protective Insulating Films of Transistor>

The insulating films 114 and 116 each have a function of supplying oxygen to the oxide semiconductor film 108. The insulating film 118 functions as a protective insulating film for the transistor 100. The insulating films 114 and 116 include oxygen. Furthermore, the insulating film 114 is an insulating film which can transmit oxygen. Note that the insulating film 114 also functions as a film which relieves damage to the oxide semiconductor film 108 at the time of forming the insulating film 116 in a later step.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm can be used as the insulating film 114.

In addition, it is preferable that the number of defects in the insulating film 114 be small and typically, the spin density corresponding to a signal that appears at g=2.001 due to a dangling bond of silicon be lower than or equal to 3×10¹⁷ spins/cm³ by electron spin resonance (ESR) measurement. This is because if the density of defects in the insulating film 114 is high, oxygen enters the defects and the amount of oxygen that passes through the insulating film 114 is decreased.

Note that all oxygen entering the insulating film 114 from the outside does not move to the outside of the insulating film 114 and some oxygen remains in the insulating film 114. Furthermore, movement of oxygen occurs in the insulating film 114 in some cases in such a manner that oxygen enters the insulating film 114 and oxygen included in the insulating film 114 moves to the outside of the insulating film 114. When an oxide insulating film which can transmit oxygen is formed as the insulating film 114, oxygen released from the insulating film 116 provided over the insulating film 114 can be moved to the oxide semiconductor film 108 through the insulating film 114.

Note that the insulating film 114 can be formed using an oxide insulating film having a low density of states due to nitrogen oxide. Note that the density of states due to nitrogen oxide can be formed between the energy of the valence band maximum (E_(v) _(_) _(os)) and the energy of the conduction band minimum (E_(c) _(_) _(os)) of the oxide semiconductor film. A silicon oxynitride film that releases less nitrogen oxide, an aluminum oxynitride film that releases less nitrogen oxide, and the like can be used as the above oxide insulating film.

Note that a silicon oxynitride film that releases less nitrogen oxide is a film of which the amount of released ammonia is larger than the amount of released nitrogen oxide in TDS analysis; the amount of released ammonia is typically greater than or equal to 1×10¹⁸/cm³ and less than or equal to 5×10¹⁹/cm³. Note that the amount of released ammonia is the amount of ammonia released by heat treatment with which the surface temperature of a film becomes higher than or equal to 50° C. and lower than or equal to 650° C., preferably higher than or equal to 50° C. and lower than or equal to 550° C.

Nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2), typically NO₂ or NO, forms levels in the insulating film 114, for example. The level is positioned in the energy gap of the oxide semiconductor film 108. Therefore, when nitrogen oxide is diffused to the interface between the insulating film 114 and the oxide semiconductor film 108, an electron is in some cases trapped by the level on the insulating film 114 side. As a result, the trapped electron remains in the vicinity of the interface between the insulating film 114 and the oxide semiconductor film 108; thus, the threshold voltage of the transistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Since nitrogen oxide included in the insulating film 114 reacts with ammonia included in the insulating film 116 in heat treatment, nitrogen oxide included in the insulating film 114 is reduced. Therefore, an electron is hardly trapped at the interface between the insulating film 114 and the oxide semiconductor film 108.

By using such an oxide insulating film, the insulating film 114 can reduce the shift in the threshold voltage of the transistor, which leads to a smaller change in the electrical characteristics of the transistor.

Note that in an ESR spectrum at 100 K or lower of the insulating film 114, by heat treatment of a manufacturing process of the transistor, typically heat treatment at a temperature higher than or equal to 300° C. and lower than 350° C., a first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, a second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and a third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 are observed. The split width of the first and second signals and the split width of the second and third signals that are obtained by ESR measurement using an X-band are each approximately 5 mT. The sum of the spin densities of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is lower than 1×10¹⁸ spins/cm³, typically higher than or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 correspond to signals attributed to nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2). Typical examples of nitrogen oxide include nitrogen monoxide and nitrogen dioxide. In other words, the lower the total spin density of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is, the lower the content of nitrogen oxide in the oxide insulating film is.

The concentration of nitrogen of the above oxide insulating film measured by SIMS is lower than or equal to 6×10²⁰ atoms/cm³.

The above oxide insulating film is formed by a PECVD method at a substrate temperature higher than or equal to 220° C. and lower than or equal to 350° C. using silane and dinitrogen monoxide, whereby a dense and hard film can be formed.

The insulating film 116 is formed using an oxide insulating film that contains oxygen in excess of that in the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film including oxygen in excess of that in the stoichiometric composition. The oxide insulating film including oxygen in excess of that in the stoichiometric composition is an oxide insulating film of which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the temperature of the film surface in the TDS analysis is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 400 nm can be used as the insulating film 116.

It is preferable that the number of defects in the insulating film 116 be small, and typically the spin density corresponding to a signal which appears at g=2.001 due to a dangling bond of silicon be lower than 1.5×10¹⁸ spins/cm³, preferably lower than or equal to 1×10¹⁸ spins/cm³ by ESR measurement. Note that the insulating film 116 is provided more apart from the oxide semiconductor film 108 than the insulating film 114 is; thus, the insulating film 116 may have higher density of defects than the insulating film 114.

Furthermore, the insulating films 114 and 116 can be formed using insulating films formed of the same kinds of materials; thus, a boundary between the insulating films 114 and 116 cannot be clearly observed in some cases. Thus, in this embodiment, the boundary between the insulating films 114 and 116 is shown by a dashed line. Although a two-layer structure of the insulating films 114 and 116 is described in this embodiment, the present invention is not limited to this. For example, a single-layer structure of the insulating film 114 may be employed.

The insulating film 118 includes nitrogen. Alternatively, the insulating film 118 includes nitrogen and silicon. The insulating film 118 has a function of blocking oxygen, hydrogen, water, alkali metal, alkaline earth metal, or the like. It is possible to prevent outward diffusion of oxygen from the oxide semiconductor film 108, outward diffusion of oxygen included in the insulating films 114 and 116, and entry of hydrogen, water, or the like into the oxide semiconductor film 108 from the outside by providing the insulating film 118. A nitride insulating film, for example, can be used as the insulating film 118. The nitride insulating film is formed using silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like. Note that instead of the nitride insulating film having a blocking effect against oxygen, hydrogen, water, alkali metal, alkaline earth metal, and the like, an oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like may be provided. As the oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like, an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, a hafnium oxynitride film, and the like can be given.

Although the variety of films such as the conductive films, the insulating films, and the oxide semiconductor films which are described above can be formed by a sputtering method or a PECVD method, such films may be formed by another method, e.g., a thermal chemical vapor deposition (CVD) method. Examples of a thermal CVD method include a metal organic chemical vapor deposition (MOCVD) method and an atomic layer deposition (ALD) method.

A thermal CVD method has an advantage that no defect due to plasma damage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a manner that a source gas and an oxidizer are supplied to the chamber at a time so that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and react with each other in the vicinity of the substrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). For example, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after the introduction of the first gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first layer; then the second source gas is introduced to react with the first layer; as a result, a second layer is stacked over the first layer, so that a thin film is formed. The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute FET.

The variety of films such as the conductive films, the insulating films, the oxide semiconductor films, and the metal oxide films in this embodiment can be formed by a thermal CVD method such as an MOCVD method or an ALD method. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium, trimethylgallium, and dimethylzinc are used. Note that the chemical formula of trimethylindium is In(CH₃)₃. The chemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formula of dimethylzinc is Zn(CH₃)₂. Without limitation to the above combination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead of trimethylgallium and diethylzinc (chemical formula: Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, that is, ozone (O₃) as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and a hafnium precursor compound (e.g., a hafnium alkoxide or a hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of another material liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, e.g., H₂O as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)) are used. Note that the chemical formula of trimethylaluminum is Al(CH₃)₃. Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by a deposition apparatus using an ALD method, hexachlorodisilane is adsorbed on a surface where a film is to be formed, chlorine included in the adsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ or dinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using a deposition apparatus using an ALD method, a WF₆ gas and a B₂H₆ gas are sequentially introduced plural times to form an initial tungsten film, and then a WF₆ gas and an H₂ gas are introduced at a time, so that a tungsten film is formed. Note that an SiH₄ gas may be used instead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., an In—Ga—Zn—O film is formed using a deposition apparatus using an ALD method, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times to form an InO layer, a Ga(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas are sequentially introduced plural times to form a ZnO layer. Note that the order of these layers is not limited to this example. A mixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing these gases. Note that although an H₂O gas which is obtained by bubbling with an inert gas such as Ar may be used instead of an O₃ gas, it is preferable to use an O₃ gas, which does not contain H. Furthermore, instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gas may be used.

Other examples of the above transistor structure are illustrated in FIGS. 39A-1, 39A-2, 39B, and 39C. A top structure of a transistor 153 illustrated in FIG. 39A-1 may be employed. FIG. 39A-2 corresponds to a cross section taken along dashed-dotted lines X1-X2 and X2-X3 in FIG. 39A-1.

Although the thickness of the oxide semiconductor film 108 b is reduced in the transistor 100 in FIGS. 1A to 1C, a transistor 154 in FIG. 39B in which the thickness of the film is not or hardly reduced may be employed. Moreover, a channel protection layer 125 may be provided as in a transistor 155 in FIG. 39C.

<Structure Example 2 of Semiconductor Device>

Structure examples different from that of the transistor 100 in FIGS. 1A to 1C are described with reference to FIGS. 3A to 3C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIG. 3A is a top view of a transistor 170 that is a semiconductor device of one embodiment of the present invention. FIG. 3B is a cross-sectional view taken along a dashed-dotted line X1-X2 illustrated in FIG. 3A, and FIG. 3C is a cross-sectional view taken along a dashed-dotted line Y1-Y2 illustrated in FIG. 3A.

The transistor 170 includes the conductive film 104 functioning as a first gate electrode over the substrate 102, the insulating film 106 over the substrate 102 and the conductive film 104, the insulating film 107 over the insulating film 106, the oxide semiconductor film 108 over the insulating film 107, the insulating film 114 over the oxide semiconductor film 108, the insulating film 116 over the insulating film 114, the conductive film 112 a functioning as a source electrode electrically connected to the oxide semiconductor film 108, the conductive film 112 b functioning as a drain electrode electrically connected to the oxide semiconductor film 108, the insulating film 114 over the conductive films 112 a and 112 b and the oxide semiconductor film 108, the insulating film 116 over the insulating film 114, the insulating film 118 over the insulating film 116, and conductive films 120 a and 120 b over the insulating film 118. The insulating films 114, 116, and 118 function as second gate insulating films of the transistor 170. The conductive film 120 a is electrically connected to the conductive film 112 b through an opening 142 c provided in the insulating films 114, 116, and 118. The conductive film 120 a in the transistor 170 functions as, for example, a pixel electrode used for a display device. The conductive film 120 b in the transistor 170 functions as a second gate electrode (also referred to as a back gate electrode).

The conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 112 b functioning as the drain electrode.

In Structure example 2, the oxide semiconductor film 108 includes a first region overlapping with the conductive film 104, a second region overlapping with the conductive film 112 a, a third region overlapping with the conductive film 112 b, and a fourth region overlapping with the conductive film 120 b. The first region is provided not to include the second region and the third region. The fourth region is provided not to include the second region and the third region. In other words, the first region is provided not to overlap with the second region and the third region, and the fourth region is provided not to overlap with the second region and the third region.

Although the conductive film 104 and the conductive film 120 b are illustrated to have the same width in FIG. 3B, the width, shape, and the like of the conductive film 120 b and those of the conductive film 104 may be the same or different.

As illustrated in FIG. 3C, the conductive film 120 b is connected to the conductive film 104 functioning as a first gate electrode through openings 142 a and 142 b provided in the insulating films 106, 107, 114, 116, and 118. Accordingly, the conductive film 120 b and the conductive film 104 are supplied with the same potential.

Note that although the structure in which the openings 142 a and 142 b are provided so that the conductive film 120 b and the conductive film 104 are connected to each other is described in this embodiment, one embodiment of the present invention is not limited thereto. For example, a structure in which only one of the openings 142 a and 142 b is provided so that the conductive film 120 b and the conductive film 104 are connected to each other, or a structure in which the openings 142 a and 142 b are not provided and the conductive film 120 b and the conductive film 104 are not connected to each other may be employed. Note that in the case where the conductive film 120 b and the conductive film 104 are not connected to each other, it is possible to apply different potentials to the conductive film 120 b and the conductive film 104.

As illustrated in FIG. 3B, the oxide semiconductor film 108 is positioned to face each of the conductive film 104 functioning as a first gate electrode and the conductive film 120 b functioning as a second gate electrode, and is sandwiched between the two conductive films functioning as gate electrodes. The length in the channel width direction of the conductive film 120 b functioning as a second gate electrode is longer than the length in the channel width direction of the oxide semiconductor film 108. The whole oxide semiconductor film 108 is covered with the conductive film 120 b with the insulating films 114, 116, and 118 positioned therebetween. Since the conductive film 120 b functioning as a second gate electrode is connected to the conductive film 104 functioning as a first gate electrode through the opening 142 a and 142 b provided in the insulating films 106, 107, 114, 116, and 118, a side surface of the oxide semiconductor film 108 in the channel width direction faces the conductive film 120 b functioning as a second gate electrode with the insulating films 114, 116, and 118 positioned therebetween.

In other words, in the channel width direction of the transistor 170, the conductive film 104 functioning as a first gate electrode and the conductive film 120 b functioning as a second gate electrode are connected to each other through the openings provided in the insulating films 106 and 107 functioning as first gate insulating films, and the insulating films 114, 116, and 118 functioning as second gate insulating films; and the conductive film 104 and the conductive film 120 b surround the oxide semiconductor film 108 with the insulating films 106 and 107 functioning as first gate insulating films, and the insulating films 114, 116, and 118 functioning as second gate insulating films positioned therebetween.

Such a structure makes it possible that the oxide semiconductor film 108 included in the transistor 170 is electrically surrounded by electric fields of the conductive film 104 functioning as a first gate electrode and the conductive film 120 b functioning as a second gate electrode. A device structure of a transistor, like that of the transistor 170, in which electric fields of a first gate electrode and a second gate electrode electrically surround an oxide semiconductor film where a channel region is formed can be referred to as a surrounded channel (s-channel) structure.

Since the transistor 170 has the s-channel structure, an electric field for inducing a channel can be effectively applied to the oxide semiconductor film 108 by the conductive film 104 functioning as a first gate electrode; therefore, the current drive capability of the transistor 170 can be improved and high on-state current characteristics can be obtained. Since the on-state current can be increased, it is possible to reduce the size of the transistor 170. In addition, since the transistor 170 is surrounded by the conductive film 104 functioning as a first gate electrode and the conductive film 120 b functioning as a second gate electrode, the mechanical strength of the transistor 170 can be increased.

Note that the other components of the transistor 170 are the same as those of the transistor 100 described above, and an effect similar to that of the transistor 100 can be obtained.

The structures of the transistors of this embodiment can be freely combined with each other. For example, the transistor 100 shown in FIGS. 1A to 1C can be used as a transistor in a pixel of a display device, and the transistor 170 shown in FIGS. 3A to 3C can be used as a transistor in a gate driver of a display device.

<Structure Example 3 of Semiconductor Device>

Although each of the first gate electrode and the second gate electrode does not overlap with the source electrode and the drain electrode in Structure example 2, a structure where the first gate electrode does not overlap with the source electrode or the drain electrode may be employed.

Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIG. 8A is a top view of a transistor 171 that is a semiconductor device of one embodiment of the present invention. FIG. 8B is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 8A. FIG. 8C is a cross-sectional view taken along a dashed-dotted line Y1-Y2 in FIG. 8A.

In FIG. 8B, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 112 b functioning as the drain electrode, while the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode and a region overlapping with the conductive film 112 b functioning as the drain electrode.

With the above transistor, parasitic capacitance between the first gate electrode and the source electrode or between the first gate electrode and the drain electrode can be reduced.

FIG. 9A is a top view of a transistor 172 that is a semiconductor device of one embodiment of the present invention. FIG. 9B is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 9A. FIG. 9C is a cross-sectional view taken along a dashed-dotted line Y1-Y2 in FIG. 9A.

In FIG. 9B, the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode and a region overlapping with the conductive film 112 b functioning as the drain electrode, while the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 112 b functioning as the drain electrode.

In Structure example 3, the oxide semiconductor film 108 includes a first region overlapping with the conductive film 104, a second region overlapping with the conductive film 112 a, a third region overlapping with the conductive film 112 b, and a fourth region overlapping with the conductive film 120 b. The first region or the fourth region is provided not to include the second region and the third region. In other words, the first region or the fourth region is provided not to overlap with the second region and the third region.

With the above structure, parasitic capacitance between the second gate electrode and the source electrode or between the second gate electrode and the drain electrode can be reduced.

Note that the other components of the transistors 171 and 172 are the same as those of the transistor 100 described above, and an effect similar to that of the transistor 100 can be obtained.

<Structure Example 4 of Semiconductor Device>

Although a bottom-gate transistor in which a first gate electrode is positioned below an oxide semiconductor film is described in Structure example 1, a top-gate transistor in which a first gate electrode is positioned above an oxide semiconductor film may be employed. A top-gate transistor 160 will be described with reference to FIGS. 10A to 10C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIG. 10A is a top view of the transistor 160 that is a semiconductor device of one embodiment of the present invention. FIG. 10B is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 10A. FIG. 10C is a cross-sectional view taken along a dashed-dotted line Y1-Y2 in FIG. 10A.

The transistor 160 includes the oxide semiconductor film 108 over the substrate 102, the conductive film 112 a functioning as a source electrode electrically connected to the oxide semiconductor film 108, the conductive film 112 b functioning as a drain electrode electrically connected to the oxide semiconductor film 108, the insulating film 107 over the conductive films 112 a and 112 b and the oxide semiconductor film 108, the insulating film 106 over the insulating film 107, and the conductive film 104 functioning as a first gate electrode over the insulating film 106. Over the transistor 160, specifically, over the insulating film 106 and the conductive films 112 a, 112 b, and 104, the insulating films 114, 116, and 118 are provided. The insulating films 114, 116, and 118 function as protective insulating films for the transistor 160.

The conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 112 b functioning as the drain electrode.

In Structure example 4, the oxide semiconductor film 108 includes a first region overlapping with the conductive film 104, a second region overlapping with the conductive film 112 a, and a third region overlapping with the conductive film 112 b. The first region is provided not to include the second region and the third region. In other words, the first region is provided not to overlap with the second region and the third region.

With the above structure, parasitic capacitance between the first gate electrode and the source electrode and between the first gate electrode and the drain electrode can be reduced.

Note that the other components of the transistor 160 are the same as those of the transistor 100 described above, and an effect similar to that of the transistor 100 can be obtained.

<Structure Example 5 of Semiconductor Device>

Although the first gate electrode does not overlap with the source electrode and the drain electrode in Structure example 1, a structure where the first gate electrode does not overlap with the source electrode or the drain electrode may be employed.

The above structure example is described with reference to FIGS. 11A-1, 11A-2, 11B-1, and 11B-2. FIGS. 11A-1 and 11B-1 are top views of transistors 151 and 152 that are semiconductor devices of embodiments of the present invention. FIG. 11A-2 is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 11A-1. FIG. 11B-2 is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 11B-1. The cross-sectional view taken along a dashed-dotted line Y1-Y2 is omitted because it is the same as that in FIG. 1C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

In FIG. 11A-2, the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode and does not overlap with the conductive film 112 b functioning as the drain electrode.

In FIG. 11B-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and has a region overlapping with the conductive film 112 b functioning as the drain electrode.

In Structure example 5, the oxide semiconductor film 108 includes a first region overlapping with the conductive film 104, a second region overlapping with the conductive film 112 a, and a third region overlapping with the conductive film 112 b. The first region is provided not to include the second region or the third region. In other words, the first region is provided not to overlap with the second region or the third region.

With the above transistor, parasitic capacitance between the first gate electrode and the source electrode or between the first gate electrode and the drain electrode can be reduced.

Note that the other components of the transistors 151 and 152 are the same as those of the transistor 100 described above, and an effect similar to that of the transistor 100 can be obtained.

<Structure Example 6 of Semiconductor Device>

Although each of the first gate electrode and the second gate electrode does not overlap with the source electrode and the drain electrode in Structure example 2, a structure where the first gate electrode and the source electrode do not overlap with each other, a structure where the first gate electrode and the source electrode, the first gate electrode and the drain electrode, the second gate electrode and the source electrode, or the second gate electrode and the drain electrode do not overlap with each other may be employed.

The above structure examples are described with reference to FIGS. 12A-1, 12A-2, 12B-1, and 12B-2, FIGS. 13A-1, 13A-2, 13B-1, and 13B-2, FIGS. 14A-1, 14A-2, 14B-1, and 14B-2, FIGS. 15A-1, 15A-2, 15B-1, and 15B-2, FIGS. 16A-1, 16A-2, 16B-1, and 16B-2, and FIGS. 17A-1, 17A-2, 17B-1, and 17B-2. FIGS. 12A-1 and 12B-1, FIGS. 13A-1 and 13B-1, FIGS. 14A-1 and 14B-1, FIGS. 15A-1 and 15B-1, FIGS. 16A-1 and 16B-1, and FIGS. 17A-1 and 17B-1 are top views of transistors 173 to 184 that are semiconductor devices of embodiments of the present invention. FIGS. 12A-2 and 12B-2, FIGS. 13A-2 and 13B-2, FIGS. 14A-2 and 14B-2, FIGS. 15A-2 and 15B-2, FIGS. 16A-2 and 16B-2, and FIGS. 17A-2 and 17B-2 are cross-sectional views taken along dashed-dotted lines X1-X2 in FIGS. 12A-1 and 12B-1, FIGS. 13A-1 and 13B-1, FIGS. 14A-1 and 14B-1, FIGS. 15A-1 and 15B-1, FIGS. 16A-1 and 16B-1, and FIGS. 17A-1 and 17B-1. The cross-sectional view taken along a dashed-dotted line Y1-Y2 is omitted because it is the same as that in FIG. 3C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

In FIG. 12A-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 112 b functioning as the drain electrode and the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 a functioning as the source electrode while the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode.

In FIG. 12B-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 112 b functioning as the drain electrode and the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 b functioning as the drain electrode while the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode.

In FIG. 13A-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 b functioning as the drain electrode and the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 112 b functioning as the drain electrode while the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode.

In FIG. 13B-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 112 b functioning as the drain electrode while the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode.

In FIG. 14A-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode, the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode, the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode, and the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode.

In FIG. 14B-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 b functioning as the drain electrode, the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode, the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode, and the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode.

In FIG. 15A-2, the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 b functioning as the drain electrode, the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode, the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode, and the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode.

In FIG. 15B-2, the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 a functioning as the source electrode, the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode, the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode, and the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode.

In FIG. 16A-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 b functioning as the drain electrode and the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 b functioning as the drain electrode while the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode and the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode.

In FIG. 16B-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 a functioning as the source electrode while the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode and the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode.

In FIG. 17A-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 b functioning as the drain electrode and the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 a functioning as the source electrode while the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode and the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode.

In FIG. 17B-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode and the conductive film 120 b functioning as the second gate electrode does not overlap with the conductive film 112 b functioning as the drain electrode while the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode and the conductive film 120 b functioning as the second gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode.

In Structure example 6, the oxide semiconductor film 108 includes a first region overlapping with the conductive film 104, a second region overlapping with the conductive film 112 a, a third region overlapping with the conductive film 112 b, and a fourth region overlapping with the conductive film 120 b. The first region or the fourth region is provided not to include the second region or the third region. In other words, the first region or the fourth region is provided not to overlap with the second region or the third region.

With the above structure, parasitic capacitance between the first gate electrode and the source electrode, between the first gate electrode and the drain electrode, between the second gate electrode and the source electrode, or between the second gate electrode and the drain electrode can be reduced.

Note that the other components of the transistors 173 to 184 are the same as those of the transistor 100 described above, and an effect similar to that of the transistor 100 can be obtained.

<Structure Example 7 of Semiconductor Device>

Although the first gate electrode of the top-gate transistor described in Structure example 4 does not overlap with the source electrode and the drain electrode, a structure where the first gate electrode does not overlap with the source electrode or the drain electrode may be employed.

The above structure example is described with reference to FIGS. 18A-1, 18A-2, 18B-1, and 18B-2. FIGS. 18A-1 and 18B-1 are top views of transistors 161 and 162 that are semiconductor devices of one embodiment of the present invention. FIG. 18A-2 is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 18A-1. FIG. 18B-2 is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 18B-1. The cross-sectional view taken along a dashed-dotted line Y1-Y2 is omitted because it is the same as that in FIG. 10C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

In FIG. 18A-1, the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 a functioning as the source electrode, and the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 b functioning as the drain electrode.

In FIG. 18B-2, the conductive film 104 functioning as the first gate electrode does not overlap with the conductive film 112 a functioning as the source electrode, and the conductive film 104 functioning as the first gate electrode has a region overlapping with the conductive film 112 b functioning as the drain electrode.

In Structure example 7, the oxide semiconductor film 108 includes a first region overlapping with the conductive film 104, a second region overlapping with the conductive film 112 a, and a third region overlapping with the conductive film 112 b. The first region is provided not to include the second region or the third region. In other words, the first region is provided not to overlap with the second region or the third region.

With the above transistor, parasitic capacitance between the first gate electrode and the source electrode or between the first gate electrode and the drain electrode can be reduced.

Note that the other components of the transistors 161 and 162 are the same as those of the transistor 100 described above, and an effect similar to that of the transistor 100 can be obtained.

<Method 1 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 100, which is a semiconductor device of one embodiment of the present invention, is described below in detail using FIGS. 4A to 4D, FIGS. 5A to 5D, and FIGS. 6A to 6C. FIGS. 4A to 4D, FIGS. 5A to 5D, and FIGS. 6A to 6C are cross-sectional views illustrating a method for manufacturing a semiconductor device.

Note that the films included in the transistor 100 (i.e., the insulating film, the oxide semiconductor film, the conductive film, and the like) can be formed by any of a sputtering method, a chemical vapor deposition (CVD) method, a vacuum evaporation method, and a pulsed laser deposition (PLD) method. Alternatively, a coating method or a printing method can be used. Although the sputtering method and a PECVD method are typical examples of the film formation method, a thermal CVD method may be used. As the thermal CVD method, an MOCVD method or an ALD method may be used, for example.

Deposition by the thermal CVD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and a source gas and an oxidizer are supplied to the chamber at a time and react with each other in the vicinity of the substrate or over the substrate. Thus, no plasma is generated in the deposition; therefore, the thermal CVD method has an advantage that no defect due to plasma damage is caused.

Deposition by an ALD method is performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). For example, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after the introduction of the first gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first single-atomic layer; then the second source gas is introduced to react with the first single-atomic layer; as a result, a second single-atomic layer is stacked over the first single-atomic layer, so that a thin film is formed.

The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute transistor.

First, a conductive film is formed over the substrate 102 and processed through a lithography process and an etching process, whereby the conductive film 104 functioning as a first gate electrode is formed. Then, the insulating films 106 and 107 functioning as first gate insulating films are formed over the conductive film 104 (see FIG. 4A).

The conductive film 104 functioning as a first gate electrode can be formed by a sputtering method, a CVD method, a vacuum evaporation method, or a PLD method. Alternatively, a coating method or a printing method can be used. Although typical deposition methods are a sputtering method and a PECVD method, a thermal CVD method, such as an MOCVD method, or an ALD method described above may be used.

In this embodiment, a glass substrate is used as the substrate 102, and as the conductive film 104 functioning as a first gate electrode, a 100-nm-thick tungsten film is formed by a sputtering method.

The insulating films 106 and 107 functioning as first gate insulating films can be formed by a sputtering method, a PECVD method, a thermal CVD method, a vacuum evaporation method, a PLD method, or the like. In this embodiment, a 400-nm-thick silicon nitride film as the insulating film 106 and a 50-nm-thick silicon oxynitride film as the insulating film 107 are formed by a PECVD method.

Note that the insulating film 106 can have a stacked-layer structure of silicon nitride films. Specifically, the insulating film 106 can have a three-layer structure of a first silicon nitride film, a second silicon nitride film, and a third silicon nitride film. An example of the three-layer structure is as follows.

For example, the first silicon nitride film can be formed to have a thickness of 50 nm under the conditions where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm are supplied as a source gas to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and the power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300 nm under the conditions where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 2000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and the power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

The third silicon nitride film can be formed to have a thickness of 50 nm under the conditions where silane at a flow rate of 200 sccm and nitrogen at a flow rate of 5000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and the power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitride film, and the third silicon nitride film can each be formed at a substrate temperature of 350° C.

When the insulating film 106 has the three-layer structure of silicon nitride films, for example, in the case where a conductive film including Cu is used as the conductive film 104, the following effect can be obtained.

The first silicon nitride film can inhibit diffusion of a copper (Cu) element from the conductive film 104. The second silicon nitride film has a function of releasing hydrogen and can improve withstand voltage of the insulating film functioning as a first gate insulating film. The third silicon nitride film releases a small amount of hydrogen and can inhibit diffusion of hydrogen released from the second silicon nitride film.

The insulating film 107 is preferably an insulating film including oxygen to improve characteristics of an interface with the oxide semiconductor film 108 (specifically the first oxide semiconductor film 108 a) formed later.

Next, the first oxide semiconductor film 108 a is formed over the insulating film 107. Then, the second oxide semiconductor film 108 b is formed over the first oxide semiconductor film 108 a (see FIG. 4B).

A method of forming the stacked-layer oxide semiconductor film in this embodiment is as follows: the first oxide semiconductor film is deposited by sputtering using an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 (atomic ratio) and then the second oxide semiconductor film is successively deposited in a vacuum by sputtering using an In—Ga—Zn metal oxide target (In:Ga:Zn=1:1:1.2 (atomic ratio). Next, a mask is formed over the stacked-layer oxide semiconductor film through a lithography process, and the stacked-layer oxide semiconductor film is processed into desired regions to form the island-shaped oxide semiconductor film 108.

In the case where the oxide semiconductor film 108 is formed by a sputtering method, as a sputtering gas, a rare gas (typically argon), oxygen, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen to a rare gas is preferably increased. In addition, increasing the purity of a sputtering gas is necessary. For example, as an oxygen gas or an argon gas used for a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, and still further preferably −120° C. or lower is used, whereby entry of moisture and the like into the oxide semiconductor film 108 can be minimized.

In the case where the oxide semiconductor film 108 is formed by a sputtering method, a chamber in a sputtering apparatus is preferably evacuated to be a high vacuum state (to the degree of about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump such as a cryopump in order to remove water or the like, which serves as an impurity for the oxide semiconductor film 108, as much as possible. Alternatively, a turbo molecular pump and a cold trap are preferably combined so as to prevent a backflow of a gas, especially a gas including carbon or hydrogen, from an exhaust system to the inside of the chamber.

Next, the conductive film 112 functioning as source and drain electrodes are formed over the insulating film 107 and the oxide semiconductor film 108 a (see FIG. 4C).

In this embodiment, as the conductive film 112, a stacked film including a 50-nm-thick tungsten film and a 400-nm-thick aluminum film is formed by a sputtering method. Although the conductive film 112 has a two-layer structure in this embodiment, one embodiment of the present invention is not limited thereto. For example, the conductive film 112 may have a three-layer structure of a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film.

Next, masks 140 a and 140 b are formed in desired regions over the conductive film 112 (see FIG. 4D).

To form the masks 140 a and 140 b in this embodiment, a photosensitive resin film is applied and is patterned by a lithography process.

Then, an etching gas 138 is applied over the conductive film 112 and the masks 140 a and 140 b to process the conductive film 112 and the second oxide semiconductor film 108 b (see FIG. 5A).

A dry-etching apparatus is used in this embodiment to process the conductive film 112 and the second oxide semiconductor film 108 b. Note that a method of forming the conductive film 112 is not limited thereto. For example, a wet-etching apparatus and a chemical solution for the etching gas 138 are used to process the conductive film 112 and the second oxide semiconductor film 108 b. However, a dry-etching apparatus is preferable than a wet-etching apparatus to process the conductive film 112 and the second oxide semiconductor film 108 b because smaller micropatterns can be formed.

Then the masks 140 a and 140 b are removed, and the conductive films 112 a and 112 b respectively serving as a source electrode and a drain electrode over the second oxide semiconductor film 108 b are formed. As the oxide semiconductor film 108, the first oxide semiconductor film 108 a and the second oxide semiconductor film 108 b having a depression are formed (see FIG. 5B).

A chemical solution may be applied over the second oxide semiconductor film 108 b and the conductive films 112 a and 112 b to clean the surface of the second oxide semiconductor film 108 b on the back-channel side. The cleaning may be performed, for example, using a chemical solution such as phosphoric acid. The cleaning using a chemical solution such as a phosphoric acid can remove impurities (e.g., elements included in the conductive films 112 a and 112 b) attached to the surface of the second oxide semiconductor film 108 b. Note that the cleaning is not necessarily performed, and thus the cleaning may be unnecessary.

Through the formation and/or cleaning of the conductive films 112 a and 112 b, the second region whose thickness is smaller than that of the first oxide semiconductor film 108 a is formed in the second oxide semiconductor film 108 b.

Next, the insulating films 114 and 116 are formed over the oxide semiconductor film 108 and the conductive films 112 a and 112 b (see FIG. 5C).

Note that after the insulating film 114 is formed, the insulating film 116 is preferably formed in succession without exposure to the air. After the insulating film 114 is formed, the insulating film 116 is formed in succession by adjusting at least one of the flow rate of a source gas, pressure, a high-frequency power, and a substrate temperature without exposure to the air, whereby the concentration of impurities attributed to the atmospheric component at the interface between the insulating film 114 and the insulating film 116 can be reduced and oxygen in the insulating films 114 and 116 can be moved to the oxide semiconductor film 108; accordingly, the amount of oxygen vacancy in the oxide semiconductor film 108 can be reduced.

For example, as the insulating film 114, a silicon oxynitride film can be formed by a PECVD method. In this case, a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas include dinitrogen monoxide and nitrogen dioxide. An insulating film including nitrogen and having a small number of defects can be formed as the insulating film 114 by a PECVD method under the conditions where the flow rate of the oxidizing gas is higher than 20 times and lower than 100 times, preferably higher than or equal to 40 times and lower than or equal to 80 times, that of the deposition gas; and the pressure in a treatment chamber is lower than 100 Pa, preferably lower than or equal to 50 Pa.

In this embodiment, a silicon oxynitride film is formed as the insulating film 114 by a PECVD method under the conditions where the substrate 102 is held at a temperature of 220° C., silane at a flow rate of 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are used as a source gas, the pressure in the treatment chamber is 20 Pa, and a high-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as the power density) is supplied to parallel-plate electrodes.

As the insulating film 116, a silicon oxide film or a silicon oxynitride film is formed under the following conditions: the substrate placed in a treatment chamber of the PECVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 350° C., the pressure is greater than or equal to 100 Pa and less than or equal to 250 Pa, preferably greater than or equal to 100 Pa and less than or equal to 200 Pa, with introduction of a source gas into the treatment chamber, and a high-frequency power of greater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm², preferably greater than or equal to 0.25 W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrode provided in the treatment chamber.

As the deposition conditions of the insulating film 116, the high-frequency power having the above power density is supplied to a reaction chamber having the above pressure, whereby the degradation efficiency of the source gas in plasma is increased, oxygen radicals are increased, and oxidation of the source gas is promoted; thus, the oxygen content in the insulating film 116 becomes higher than that in the stoichiometric composition. In the film formed at a substrate temperature within the above temperature range, however, the bond between silicon and oxygen is weak, and part of oxygen in the film is released by heat treatment in a later step. Thus, it is possible to form an oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition and from which part of oxygen is released by heating.

The step of forming the insulating film 116 is performed using a PECVD apparatus at a temperature higher than or equal to 180° C. and lower than or equal to 350° C., and a temperature in a step of forming the insulating film 116 is preferably the highest in the manufacturing process of the transistor 100. For example, formation of the insulating film 116 at 350° C. enables the transistor 100 to be directly formed on a flexible substrate or the like.

Note that the insulating film 114 functions as a protective film for the oxide semiconductor film 108 in the step of forming the insulating film 116. Therefore, the insulating film 116 can be formed using the high-frequency power having a high power density while damage to the oxide semiconductor film 108 is reduced.

Note that in the deposition conditions of the insulating film 116, when the flow rate of the deposition gas containing silicon with respect to the oxidizing gas is increased, the amount of defects in the insulating film 116 can be reduced. Typically, it is possible to form an oxide insulating layer in which the amount of defects is small, i.e., the spin density of a signal which appears at g=2.001 originating from a dangling bond of silicon is lower than 6×10¹⁷ spins/cm³, preferably lower than or equal to 3×10¹⁷ spins/cm³, further preferably lower than or equal to 1.5×10¹⁷ spins/cm³ by ESR measurement. As a result, the reliability of the transistor can be improved.

Heat treatment may be performed after the insulating films 114 and 116 are formed. The heat treatment can reduce nitrogen oxide included in the insulating films 114 and 116. By the heat treatment, part of oxygen included in the insulating films 114 and 116 can be moved to the oxide semiconductor film 108, so that the amount of oxygen vacancy included in the oxide semiconductor film 108 can be reduced.

The temperature of the heat treatment on the insulating films 114 and 116 is typically higher than or equal to 150° C. and lower than or equal to 350° C. The heat treatment can be performed under an atmosphere of nitrogen, oxygen, ultra-dry air (air with a water content of 20 ppm or less, preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas (e.g., argon or helium). The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas preferably does not contain hydrogen, water, and the like. An electric furnace, an RTA apparatus, or the like can be used for the heat treatment.

In this embodiment, the heat treatment is performed at 350° C. in a nitrogen atmosphere for 1 hour. Note that the temperature at the formation step of the insulating film 116 is the highest of temperatures in the formation process of the transistor 100, and heat treatment at temperatures equivalent to the temperature for forming the insulating film 116 may be performed in other steps.

Next, an oxide conductive film 131 is formed over the insulating film 116 (see FIG. 5D).

The oxide conductive film 131 contains oxygen and metal (at least one of indium, zinc, titanium, aluminum, tungsten, tantalum, and molybdenum).

The oxide conductive film 131 can be formed using, for example, a tantalum oxynitride film, a titanium oxide film, an indium tin oxide (hereinafter also referred to as ITO) film, an aluminum oxide film, or an oxide semiconductor film (e.g., an IGZO film having an atomic ratio of In:Ga:Zn=1:4:5). The oxide conductive film 131 can be formed by a sputtering method, for example. The thickness of the oxide conductive film 131 is preferably greater than or equal to 1 nm and less than or equal to 20 nm, or greater than or equal to 2 nm and less than or equal to 10 nm. In this embodiment, a 5-nm-thick indium tin oxide doped with silicon oxide (hereinafter referred to as ITSO) is used for the oxide conductive film 131.

Next, the oxygen 139 is added to the insulating films 114 and 116 and the oxide semiconductor film 108 through the oxide conductive film 131. In FIG. 6A, oxygen added to the insulating films 114 and 116 is schematically shown as oxygen 139.

As a method for adding the oxygen 139 to the insulating films 114 and 116 and the oxide semiconductor film 108 through the oxide conductive film 131, an ion doping method, an ion implantation method, plasma treatment, or the like is given. By the bias application to the substrate side when the oxygen 139 is added, the oxygen 139 can be effectively added to the insulating films 114 and 116 and the oxide semiconductor film 108. As the bias, for example, power density can be greater than or equal to 1 W/cm² and less than or equal to 5 W/cm². When the oxide conductive film 131 is provided over the insulating film 116 and then oxygen is added, the oxide conductive film 131 functions as a protective film for inhibiting release of oxygen from the insulating film 116. Thus, a larger amount of oxygen can be added to the insulating films 114 and 116 and the oxide semiconductor film 108.

The oxide conductive film 131 is removed using an etchant 142 (see FIG. 6B).

As a method for removing the oxide conductive film 131, for example, a dry etching method, a wet etching method, or a combination a dry etching method and a wet etching method can be used. Note that the etchant 142 is an etching gas in the case of dry etching and is a chemical solution in the case of wet etching. In this embodiment, a wet etching method is used to remove the oxide conductive film 131.

Next, the insulating film 118 is formed over the insulating film 116 (see FIG. 6C).

Note that heat treatment may be performed before or after the formation of the insulating film 118, so that excess oxygen included in the insulating films 114 and 116 can diffuse into the oxide semiconductor film 108 to fill oxygen vacancy in the oxide semiconductor film 108. Alternatively, the insulating film 118 may be deposited by heating, so that excess oxygen included in the insulating films 114 and 116 can diffuse into the oxide semiconductor film 108 to fill oxygen vacancy in the oxide semiconductor film 108.

In the case of using a PECVD method, the substrate temperature is preferably set to higher than or equal to 180° C. and lower than or equal to 350° C. because a dense insulating film 118 can be formed.

For example, in the case where a silicon nitride film is formed by a PECVD method as the insulating film 118, a deposition gas containing silicon, nitrogen, and ammonia are preferably used as a source gas. A small amount of ammonia compared to the amount of nitrogen is used, whereby ammonia is dissociated in the plasma and activated species are generated. The activated species break a bond between silicon and hydrogen that are contained in a deposition gas containing silicon and a triple bond between nitrogen molecules. As a result, a dense silicon nitride film having few defects, in which bonds between silicon and nitrogen are promoted and bonds between silicon and hydrogen is few, can be formed. On the other hand, when the amount of ammonia with respect to nitrogen is large, decomposition of a deposition gas containing silicon and decomposition of nitrogen are not promoted, so that a sparse silicon nitride film in which bonds between silicon and hydrogen remain and defects are increased is formed. Therefore, in the source gas, a flow rate ratio of the nitrogen to the ammonia is set to be greater than or equal to 5 and less than or equal to 50, preferably greater than or equal to 10 and less than or equal to 50.

In this embodiment, with the use of a PECVD apparatus, a 50-nm-thick silicon nitride film is formed as the insulating film 118 using silane, nitrogen, and ammonia as a source gas. The flow rate of silane is 50 sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate of ammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa, the substrate temperature is 350° C., and high-frequency power of 1000 W is supplied to parallel-plate electrodes with a 27.12 MHz high-frequency power source. Note that the PECVD apparatus is a parallel-plate PECVD apparatus in which the electrode area is 6000 cm², and the power per unit area (power density) into which the supplied power is converted is 1.7×10⁻¹ W/cm².

Through the above process, the transistor 100 illustrated in FIGS. 1A to 1C can be manufactured.

The L_(off) transistor and the L_(ov) transistor can be separately fabricated by changing the pattern of the first gate electrode, the oxide semiconductor film, the source electrode, or the drain electrode; thus, the transistors can be formed over the same substrate by the same manufacturing method at the same time.

Although a structure in which the first gate electrode does not overlap with the source electrode and the drain electrode is described as an example in this manufacturing method, a structure where the first gate electrode does not overlap with the source electrode or the drain electrode can be formed by a similar manufacturing method.

<Method 2 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 170, which is one embodiment of the present invention, is described in detail with reference to FIGS. 7A to 7H. FIGS. 7A to 7H are cross-sectional views illustrating the method for manufacturing the semiconductor device. FIGS. 7A, 7C, 7E, and 7G are cross-sectional views in the channel length direction of the transistor 170. FIGS. 7B, 7D, 7F, and 7H are cross-sectional views in the channel width direction of the transistor 170.

First, the conductive film 104, the insulating films 106 and 107, the oxide semiconductor film 108, the conductive films 112 a and 112 b, and the insulating films 114, 116, and 118 are formed over the substrate 102 (see FIGS. 7A and 7B) through steps similar to the steps in the manufacturing method of the transistor 100 (the steps shown in FIGS. 4A to 4D, FIGS. 5A to 5D, and FIGS. 6A to 6C).

Next, a mask is formed over the insulating film 118 through a lithography process, and the opening 142 c is formed in a desired region in the insulating films 114, 116, and 118. In addition, a mask is formed over the insulating film 118 through a lithography process, and the openings 142 a and 142 b are formed in desired regions in the insulating films 106, 107, 114, 116, and 118. Note that the opening 142 c reaches the conductive film 112 b. The openings 142 a and 142 b reach the conductive film 104 (see FIGS. 7C and 7D).

Note that the openings 142 a and 142 b and the opening 142 c may be formed in the same step or may be formed by different steps. In the case where the openings 142 a and 142 b and the opening 142 c are formed in the same step, for example, a gray-tone mask or a half-tone mask can be used. Moreover, the openings 142 a and 142 b may be formed in some steps. For example, the insulating films 106 and 107 are processed and then the insulating films 114, 116, and 118 are processed.

Next, a conductive film 120 is formed over the insulating film 118 to cover the openings 142 a, 142 b, and 142 c (see FIGS. 7E and 7F).

For the conductive film 120, for example, a material including one of indium (In), zinc (Zn), and tin (Sn) can be used. In particular, for the conductive film 120, a light-transmitting conductive material such as indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium tin oxide (ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added (ITSO) can be used. The conductive film 120 can be formed by a sputtering method, for example. In this embodiment, a 110-nm-thick ITSO film is formed by a sputtering method.

Next, a mask is formed over the conductive film 120 through a lithography process, and the conductive film 112 is processed into a desired shape to form the conductive films 120 a and 120 b (see FIGS. 7G and 7H).

A method for forming the conductive films 120 a and 120 b is, for example, a dry etching method, a wet etching method, or a combination of a dry etching method and a wet etching method. A wet etching method is used in this embodiment to process the conductive film 120 into the conductive films 120 a and 120 b.

Through the above process, the transistor 170 illustrated in FIGS. 3A to 3C can be manufactured.

The L_(off) transistor and the L_(ov) transistor can be separately fabricated by changing the pattern of the first gate electrode, the second gate electrode, the oxide semiconductor film, the source electrode, or the drain electrode; thus, the transistors can be formed over the same substrate by the same manufacturing method at the same time.

Although the structure where each of the first gate electrode and the second gate electrode does not overlap with the source electrode and the drain electrode is described as an example in this manufacturing method, a structure where the first gate electrode or the second gate electrode does not overlap with the source electrode and the drain electrode can be formed by a similar manufacturing method. Furthermore, a structure where the first gate electrode and the source electrode, the first gate electrode and the drain electrode, the second gate electrode and the source electrode, or the second gate electrode and the drain electrode do not overlap with each other can be formed by a similar manufacturing method.

The structure and method described in this embodiment can be implemented by being combined as appropriate with any of the other structures and methods described in the other embodiments.

Embodiment 2

In this embodiment, the structure of an oxide semiconductor included in a semiconductor device of one embodiment of the present invention will be described in detail.

<Structure of Oxide Semiconductor>

An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.

From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of a crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS.

It is known that an amorphous structure is generally defined as being metastable and unfixed, and being isotropic and having no non-uniform structure. In other words, an amorphous structure has a flexible bond angle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor. Moreover, an oxide semiconductor that is not isotropic (e.g., an oxide semiconductor that has a periodic structure in a microscopic region) cannot be regarded as a completely amorphous oxide semiconductor. Note that an a-like OS has a periodic structure in a microscopic region, but at the same time has a void and has an unstable structure. For this reason, an a-like OS has physical properties similar to those of an amorphous oxide semiconductor.

<Caac-Os>

First, a CAAC-OS will be described.

The CAAC-OS is one of oxide semiconductors having a plurality of c-axis aligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a transmission electron microscope (TEM), a plurality of pellets can be observed. However, in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur.

A CAAC-OS observed with TEM will be described below. FIG. 19A shows a high-resolution TEM image of a cross section of the CAAC-OS which is observed from a direction substantially parallel to the sample surface. The high-resolution TEM image is obtained with a spherical aberration corrector function. The high-resolution TEM image obtained with a spherical aberration corrector function is particularly referred to as a Cs-corrected high-resolution TEM image. The Cs-corrected high-resolution TEM image can be obtained with, for example, an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 19B is an enlarged Cs-corrected high-resolution TEM image of a region (1) in FIG. 19A. FIG. 19B shows that metal atoms are arranged in a layered manner in a pellet. Each metal atom layer has a configuration reflecting unevenness of a surface over which a CAAC-OS film is formed (hereinafter, the surface is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged parallel to the formation surface or the top surface of the CAAC-OS film.

As shown in FIG. 19B, the CAAC-OS has a characteristic atomic arrangement. The characteristic atomic arrangement is denoted by an auxiliary line in FIG. 19C. FIGS. 19B and 19C prove that the size of a pellet is greater than or equal to 1 nm or greater than or equal to 3 nm, and the size of a space caused by tilt of the pellets is approximately 0.8 nm. Therefore, the pellet can also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OS can also be referred to as an oxide semiconductor including c-axis aligned nanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, the schematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120 is illustrated by such a structure in which bricks or blocks are stacked (see FIG. 19D). The part in which the pellets are tilted as observed in FIG. 19C corresponds to a region 5161 shown in FIG. 19D.

FIG. 20A shows a Cs-corrected high-resolution TEM image of a plane of the CAAC-OS observed from a direction substantially perpendicular to the sample surface. FIGS. 20B, 20C, and 20D are enlarged Cs-corrected high-resolution TEM images of regions (1), (2), and (3) in FIG. 20A, respectively. FIGS. 20B, 20C, and 20D indicate that metal atoms are arranged in a triangular, quadrangular, or hexagonal configuration in a pellet. However, there is no regularity of arrangement of metal atoms between different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) will be described. For example, when the structure of a CAAC-OS including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2θ) of around 31° as shown in FIG. 21A. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-plane method, another peak may appear when 2θ is around 36°, in addition to the peak at 2θ of around 31°. The peak at 2θ of around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS. It is preferable that in the CAAC-OS analyzed by an out-of-plane method, a peak appear when 2θ is around 31° and that a peak not appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray beam is incident on a sample in a direction substantially perpendicular to the c-axis, a peak appears when 2θ is around 56°. This peak is attributed to the (110) plane of the InGaZnO₄ crystal. In the case of the CAAC-OS, when analysis (φ scan) is performed with 2θ fixed at around 56° and with the sample rotated using a normal vector of the sample surface as an axis (φ axis), as shown in FIG. 21B, a peak is not clearly observed. In contrast, in the case of a single crystal oxide semiconductor of InGaZnO₄, when φ scan is performed with 2θ fixed at around 56°, as shown in FIG. 21C, six peaks which are derived from crystal planes equivalent to the (110) plane are observed. Accordingly, the structural analysis using XRD shows that the directions of a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction will be described. For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO₄ crystal in a direction parallel to the sample surface, a diffraction pattern (also referred to as a selected-area transmission electron diffraction pattern) shown in FIG. 22A can be obtained. In this diffraction pattern, spots derived from the (009) plane of an InGaZnO₄ crystal are included. Thus, the electron diffraction also indicates that pellets included in the CAAC-OS have c-axis alignment and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile, FIG. 22B shows a diffraction pattern obtained in such a manner that an electron beam with a probe diameter of 300 nm is incident on the same sample in a direction perpendicular to the sample surface. As shown in FIG. 22B, a ring-like diffraction pattern is observed. Thus, the electron diffraction also indicates that the a-axes and b-axes of the pellets included in the CAAC-OS do not have regular alignment. The first ring in FIG. 22B is considered to be derived from the (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal. The second ring in FIG. 22B is considered to be derived from the (110) plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancies).

Note that the impurity means an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities or defects might be changed by light, heat, or the like. For example, impurities contained in the oxide semiconductor might serve as carrier traps or carrier generation sources. Furthermore, oxygen vacancies in the oxide semiconductor serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies is an oxide semiconductor with a low carrier density. Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. Thus, the CAAC-OS can be referred to as an oxide semiconductor having stable characteristics.

<Nc-OS>

Next, an nc-OS will be described.

An nc-OS has a region in which a crystal part is observed and a region in which a crystal part is not clearly observed in a high-resolution TEM image. In most cases, the size of a crystal part included in the nc-OS is greater than or equal to 1 nm and less than or equal to 10 nm, or greater than or equal to 1 nm. Note that an oxide semiconductor including a crystal part whose size is greater than 10 nm and less than or equal to 100 nm is sometimes referred to as a microcrystalline oxide semiconductor. In a high-resolution TEM image of the nc-OS, for example, a grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as a pellet in the following description.

In the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method. For example, when the nc-OS is analyzed by an out-of-plane method using an X-ray beam having a diameter larger than the size of a pellet, a peak which shows a crystal plane does not appear. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS is subjected to electron diffraction using an electron beam with a probe diameter (e.g., 50 nm or larger) that is larger than the size of a pellet. Meanwhile, spots appear in a nanobeam electron diffraction pattern of the nc-OS when an electron beam having a probe diameter close to or smaller than the size of a pellet is applied. Moreover, in a nanobeam electron diffraction pattern of the nc-OS, bright regions in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS, a plurality of spots are shown in a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets (nanocrystals) as mentioned above, the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as compared with an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an a-like OS and an amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS.

<A-Like OS>

An a-like OS has a structure between those of the nc-OS and the amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed.

The a-like OS has an unstable structure because it includes a void. To verify that an a-like OS has an unstable structure as compared with a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below.

An a-like OS (referred to as Sample A), an nc-OS (referred to as Sample B), and a CAAC-OS (referred to as Sample C) are prepared as samples subjected to electron irradiation. Each of the samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample is obtained. The high-resolution cross-sectional TEM images show that all the samples have crystal parts.

Note that which part is regarded as a crystal part is determined as follows. It is known that a unit cell of an InGaZnO₄ crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are stacked in the c-axis direction. The distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion where the lattice spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄. Each of lattice fringes corresponds to the a-b plane of the InGaZnO₄ crystal.

FIG. 23 shows change in the average size of crystal parts (at 22 points to 45 points) in each sample. Note that the crystal part size corresponds to the length of a lattice fringe. FIG. 23 indicates that the crystal part size in the a-like OS increases with an increase in the cumulative electron dose. Specifically, as shown by (1) in FIG. 23, a crystal part of approximately 1.2 nm (also referred to as an initial nucleus) at the start of TEM observation grows to a size of approximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OS shows little change from the start of electron irradiation to a cumulative electron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3) in FIG. 23, the average crystal sizes in an nc-OS and a CAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively, regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. Therefore, the a-like OS has an unstable structure as compared with the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS because it includes a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of each of the nc-OS and the CAAC-OS is higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. It is difficult to deposit an oxide semiconductor having a density of lower than 78% of the density of the single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with a rhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm³ and lower than 5.9 g/cm³. For example, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of each of the nc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having a certain composition cannot exist in a single crystal structure. In that case, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition can be calculated using a weighted average according to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density.

As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked layer including two or more films of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS, for example.

<Deposition Model>

Examples of deposition models of a CAAC-OS and an nc-OS are described below.

FIG. 24A is a schematic view of the inside of a deposition chamber where a CAAC-OS is deposited by a sputtering method.

A target 5130 is attached to a backing plate. A plurality of magnets are provided to face the target 5130 with the backing plate positioned therebetween. The plurality of magnets generate a magnetic field. The above description on the deposition chamber is referred to for the layout and structure of magnets. A sputtering method in which the deposition rate is increased by utilizing a magnetic field of magnets is referred to as a magnetron sputtering method.

The target 5130 has a polycrystalline structure in which a cleavage plane exists in at least one crystal grain.

A cleavage plane of the target 5130 including an In—Ga—Zn oxide is described as an example. FIG. 25A shows a structure of an InGaZnO₄ crystal included in the target 5130. Note that FIG. 25A shows a structure of the case where the InGaZnO₄ crystal is observed from a direction parallel to the b-axis when the c-axis is in an upward direction.

FIG. 25A indicates that oxygen atoms in a Ga—Zn—O layer are positioned close to those in an adjacent Ga—Zn—O layer. The oxygen atoms have negative charge, whereby the two Ga—Zn—O layers repel each other. As a result, the InGaZnO₄ crystal has a cleavage plane between the two adjacent Ga—Zn—O layers.

The substrate 5120 is placed to face the target 5130, and the distance d (also referred to as a target-substrate distance (T-S distance)) is greater than or equal to 0.01 m and less than or equal to 1 m, preferably greater than or equal to 0.02 m and less than or equal to 0.5 m. The deposition chamber is mostly filled with a deposition gas (e.g., an oxygen gas, an argon gas, or a mixed gas containing oxygen at 5 vol % or higher) and the pressure in the deposition chamber is controlled to be higher than or equal to 0.01 Pa and lower than or equal to 100 Pa, preferably higher than or equal to 0.1 Pa and lower than or equal to 10 Pa. Here, discharge starts by application of a voltage at a certain value or higher to the target 5130, and plasma is observed. The magnetic field forms a high-density plasma region in the vicinity of the target 5130. In the high-density plasma region, the deposition gas is ionized, so that an ion 5101 is generated. Examples of the ion 5101 include an oxygen cation (O₊) and an argon cation (Ar⁺).

The ion 5101 is accelerated toward the target 5130 side by an electric field, and then collides with the target 5130. At this time, a pellet 5100 a and a pellet 5100 b which are flat-plate-like (pellet-like) sputtered particles are separated and sputtered from the cleavage plane. Note that structures of the pellet 5100 a and the pellet 5100 b may be distorted by an impact of collision of the ion 5101.

The pellet 5100 a is a flat-plate-like (pellet-like) sputtered particle having a triangle plane, e.g., regular triangle plane. The pellet 5100 b is a flat-plate-like (pellet-like) sputtered particle having a hexagon plane, e.g., regular hexagon plane. Note that flat-plate-like (pellet-like) sputtered particles such as the pellet 5100 a and the pellet 5100 b are collectively called pellets 5100. The shape of a flat plane of the pellet 5100 is not limited to a triangle or a hexagon. For example, the flat plane may have a shape formed by combining two or more triangles. For example, a quadrangle (e.g., rhombus) may be formed by combining two triangles (e.g., regular triangles).

The thickness of the pellet 5100 is determined depending on the kind of deposition gas and the like. The thicknesses of the pellets 5100 are preferably uniform; the reason for this is described later. In addition, the sputtered particle preferably has a pellet shape with a small thickness as compared to a dice shape with a large thickness. For example, the thickness of the pellet 5100 is greater than or equal to 0.4 nm and less than or equal to 1 nm, preferably greater than or equal to 0.6 nm and less than or equal to 0.8 nm. In addition, the width of the pellet 5100 is, for example, greater than or equal to 1 nm. The pellet 5100 corresponds to the initial nucleus in the description of (1) in FIG. 23. For example, in the case where the ion 5101 collides with the target 5130 including an In—Ga—Zn oxide, the pellet 5100 that includes three layers of a Ga—Zn—O layer, an In—O layer, and a Ga—Zn—O layer as shown in FIG. 25B is ejected. Note that FIG. 25C shows the structure of the pellet 5100 observed from a direction parallel to the c-axis. Therefore, the pellet 5100 has a nanometer-sized sandwich structure including two Ga—Zn—O layers (pieces of bread) and an In—O layer (filling).

The pellet 5100 may receive a charge when passing through the plasma, so that side surfaces thereof are negatively or positively charged. The pellet 5100 includes an oxygen atom on its side surface, and the oxygen atom may be negatively charged. In this manner, when the side surfaces are charged with the same polarity, charges repel each other, and accordingly, the pellet 5100 can maintain a flat-plate shape. In the case where a CAAC-OS is an In—Ga—Zn oxide, there is a possibility that an oxygen atom bonded to an indium atom is negatively charged. There is another possibility that an oxygen atom bonded to an indium atom, a gallium atom, or a zinc atom is negatively charged. In addition, the pellet 5100 may grow by being bonded with an indium atom, a gallium atom, a zinc atom, an oxygen atom, or the like when passing through plasma. This is a cause of a difference in size between (2) and (1) in FIG. 23. Here, in the case where the temperature of the substrate 5120 is at around room temperature, the pellet 5100 does not grow anymore; thus, an nc-OS is formed (see FIG. 24B). An nc-OS can be deposited when the substrate 5120 has a large size because a temperature at which the deposition of an nc-OS is carried out is approximately room temperature. Note that in order that the pellet 5100 grows in plasma, it is effective to increase deposition power in sputtering. High deposition power can stabilize the structure of the pellet 5100.

As shown in FIGS. 24A and 24B, the pellet 5100 flies like a kite in plasma and flutters up to the substrate 5120. Since the pellets 5100 are charged, when the pellet 5100 gets close to a region where another pellet 5100 has already been deposited, repulsion is generated. Here, above the substrate 5120, a magnetic field in a direction parallel to the top surface of the substrate 5120 (also referred to as a horizontal magnetic field) is generated. A potential difference is given between the substrate 5120 and the target 5130, and accordingly, current flows from the substrate 5120 toward the target 5130. Thus, the pellet 5100 is given a force (Lorentz force) on the top surface of the substrate 5120 by an effect of the magnetic field and the current. This is explainable with Fleming's left-hand rule.

The mass of the pellet 5100 is larger than that of an atom. Therefore, to move the pellet 5100 over the top surface of the substrate 5120, it is important to apply some force to the pellet 5100 from the outside. One kind of the force may be force which is generated by the action of a magnetic field and current. In order to increase a force applied to the pellet 5100, it is preferable to provide, on the top surface, a region where the magnetic field in a direction parallel to the top surface of the substrate 5120 is 10 G or higher, preferably 20 G or higher, further preferably 30 G or higher, still further preferably 50 G or higher. Alternatively, it is preferable to provide, on the top surface, a region where the magnetic field in a direction parallel to the top surface of the substrate 5120 is 1.5 times or higher, preferably twice or higher, further preferably 3 times or higher, still further preferably 5 times or higher as high as the magnetic field in a direction perpendicular to the top surface of the substrate 5120.

At this time, magnet units and/or the substrate 5120 are moved or rotated relatively, whereby the direction of the horizontal magnetic field on the top surface of the substrate 5120 continues to change. Therefore, the pellet 5100 can be moved in various directions on the top surface of the substrate 5120 by receiving forces in various directions.

Furthermore, as shown in FIG. 24A, when the substrate 5120 is heated, resistance between the pellet 5100 and the substrate 5120 due to friction or the like is low. As a result, the pellet 5100 glides above the top surface of the substrate 5120. The glide of the pellet 5100 is caused in a state where its flat plane faces the substrate 5120. Then, when the pellet 5100 reaches the side surface of another pellet 5100 that has been already deposited, the side surfaces of the pellets 5100 are bonded. At this time, the oxygen atom on the side surface of the pellet 5100 is released. With the released oxygen atom, oxygen vacancies in a CAAC-OS might be filled; thus, the CAAC-OS has a low density of defect states. Note that the temperature of the top surface of the substrate 5120 is, for example, higher than or equal to 100° C. and lower than 500° C., higher than or equal to 150° C. and lower than 450° C., higher than or equal to 170° C. and lower than 400° C., or higher than or equal to 170° C. and lower than or equal to 350° C. Hence, even when the substrate 5120 has a large size, it is possible to deposit a CAAC-OS.

Furthermore, the pellet 5100 is heated on the substrate 5120, whereby atoms are rearranged, and the structure distortion caused by the collision of the ion 5101 can be reduced. The pellet 5100 whose structure distortion is reduced is substantially single crystal. Even when the pellets 5100 are heated after being bonded, expansion and contraction of the pellet 5100 itself hardly occur, which is caused by turning the pellet 5100 into substantially single crystal. Thus, formation of defects such as a grain boundary due to expansion of a space between the pellets 5100 can be prevented, and accordingly, generation of crevasses can be prevented.

The CAAC-OS does not have a structure like a board of a single crystal oxide semiconductor but has arrangement with a group of pellets 5100 (nanocrystals) like stacked bricks or blocks. Furthermore, a grain boundary does not exist therebetween. Therefore, even when deformation such as shrink occurs in the CAAC-OS owing to heating during deposition, heating or bending after deposition, it is possible to relieve local stress or release distortion. Therefore, this structure is suitable for a flexible semiconductor device. Note that the nc-OS has arrangement in which pellets 5100 (nanocrystals) are randomly stacked.

When the target is sputtered with an ion, in addition to the pellets, zinc oxide or the like may be ejected. The zinc oxide is lighter than the pellet and thus reaches the top surface of the substrate 5120 before the pellet. As a result, the zinc oxide forms a zinc oxide layer 5102 with a thickness greater than or equal to 0.1 nm and less than or equal to 10 nm, greater than or equal to 0.2 nm and less than or equal to 5 nm, or greater than or equal to 0.5 nm and less than or equal to 2 nm. FIGS. 26A to 26D are cross-sectional schematic views.

As illustrated in FIG. 26A, a pellet 5105 a and a pellet 5105 b are deposited over the zinc oxide layer 5102. Here, side surfaces of the pellet 5105 a and the pellet 5105 b are in contact with each other. In addition, a pellet 5105 c is deposited over the pellet 5105 b, and then glides over the pellet 5105 b. Furthermore, a plurality of particles 5103 ejected from the target together with the zinc oxide are crystallized by heating of the substrate 5120 to form a region 5105 a 1 on another side surface of the pellet 5105 a. Note that the plurality of particles 5103 may contain oxygen, zinc, indium, gallium, or the like.

Then, as illustrated in FIG. 26B, the region 5105 a 1 grows to part of the pellet 5105 a to form a pellet 5105 a 2. In addition, a side surface of the pellet 5105 c is in contact with another side surface of the pellet 5105 b.

Next, as illustrated in FIG. 26C, a pellet 5105 d is deposited over the pellet 5105 a 2 and the pellet 5105 b, and then glides over the pellet 5105 a 2 and the pellet 5105 b. Furthermore, a pellet 5105 e glides toward another side surface of the pellet 5105 c over the zinc oxide layer 5102.

Then, as illustrated in FIG. 26D, the pellet 5105 d is placed so that a side surface of the pellet 5105 d is in contact with a side surface of the pellet 5105 a 2. Furthermore, a side surface of the pellet 5105 e is in contact with another side surface of the pellet 5105 c. A plurality of particles 5103 ejected from the target together with the zinc oxide are crystallized by heating of the substrate 5120 to form a region 5105 d 1 on another side surface of the pellet 5105 d.

As described above, deposited pellets are placed to be in contact with each other and then growth is caused at side surfaces of the pellets, whereby a CAAC-OS is formed over the substrate 5120. Therefore, each pellet of the CAAC-OS is larger than that of the nc-OS. A difference in size between (3) and (2) in FIG. 23 corresponds to the amount of growth after deposition.

When spaces between pellets 5100 are extremely small, the pellets may form a large pellet. The large pellet has a single crystal structure. For example, the size of the large pellet may be greater than or equal to 10 nm and less than or equal to 200 nm, greater than or equal to 15 nm and less than or equal to 100 nm, or greater than or equal to 20 nm and less than or equal to 50 nm, when seen from the above. Therefore, when a channel formation region of a transistor is smaller than the large pellet, the region having a single crystal structure can be used as the channel formation region. Furthermore, when the size of the pellet is increased, the region having a single crystal structure can be used as the channel formation region, the source region, and the drain region of the transistor.

In this manner, when the channel formation region or the like of the transistor is formed in a region having a single crystal structure, the frequency characteristics of the transistor can be increased in some cases.

As shown in such a model, the pellets 5100 are considered to be deposited on the substrate 5120. Thus, a CAAC-OS can be deposited even when a formation surface does not have a crystal structure, which is different from film deposition by epitaxial growth. For example, even when the top surface (formation surface) of the substrate 5120 has an amorphous structure (e.g., the top surface is formed of amorphous silicon oxide), a CAAC-OS can be formed.

In addition, it is found that in formation of the CAAC-OS, the pellets 5100 are arranged in accordance with the top surface shape of the substrate 5120 that is the formation surface even when the formation surface has unevenness. For example, in the case where the top surface of the substrate 5120 is flat at the atomic level, the pellets 5100 are arranged so that flat planes parallel to the a-b plane face downwards. By stacking n layers (n is a natural number), the CAAC-OS can be obtained.

In the case where the top surface of the substrate 5120 has unevenness, a CAAC-OS in which n layers (n is a natural number) in each of which the pellets 5100 are arranged along the unevenness are stacked is formed. Since the substrate 5120 has unevenness, a gap is easily generated between the pellets 5100 in the CAAC-OS in some cases. Note that owing to intermolecular force, the pellets 5100 are arranged so that a gap between the pellets is as small as possible even on the unevenness surface. Therefore, even when the formation surface has unevenness, a CAAC-OS with high crystallinity can be obtained.

As a result, laser crystallization is not needed for formation of a CAAC-OS, and a uniform film can be formed even over a large-sized glass substrate or the like.

Since a CAAC-OS is deposited in accordance with such a model, the sputtered particle preferably has a pellet shape with a small thickness. Note that when the sputtered particles have a dice shape with a large thickness, planes facing the substrate 5120 vary; thus, the thicknesses and orientations of the crystals cannot be uniform in some cases.

According to the deposition model described above, a CAAC-OS with high crystallinity can be formed even on a formation surface with an amorphous structure.

The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments.

Embodiment 3

In this embodiment, an example of a display device that includes any of the transistors described in the above embodiment will be described below with reference to FIG. 27, FIG. 28, and FIG. 29.

FIG. 27 is a top view of an example of a display device. A display device 700 illustrated in FIG. 27 includes a pixel portion 702 provided over a first substrate 701; a source driver circuit portion 704 and a gate driver circuit portion 706 provided over the first substrate 701; a sealant 712 provided to surround the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706; and a second substrate 705 provided to face the first substrate 701. The first substrate 701 and the second substrate 705 are sealed with the sealant 712. That is, the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 are sealed with the first substrate 701, the sealant 712, and the second substrate 705. Although not illustrated in FIG. 27, a display element is provided between the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminal portion 708 electrically connected to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 is provided in a region different from the region which is surrounded by the sealant 712 and positioned over the first substrate 701. Furthermore, an FPC 716 is connected to the FPC terminal portion 708, and a variety of signals and the like are supplied to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 through the FPC 716. Furthermore, a signal line 710 is connected to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708. Various signals and the like are applied to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708 via the signal line 710 from the FPC 716.

A plurality of gate driver circuit portions 706 may be provided in the display device 700. An example of the display device 700 in which the source driver circuit portion 704 and the gate driver circuit portion 706 are formed over the first substrate 701 where the pixel portion 702 is also formed is described; however, the structure is not limited thereto. For example, only the gate driver circuit portion 706 may be formed over the first substrate 701 or only the source driver circuit portion 704 may be formed over the first substrate 701. In this case, a substrate where a source driver circuit, a gate driver circuit, or the like is formed (e.g., a driver-circuit substrate formed using a single-crystal semiconductor film or a polycrystalline semiconductor film) may be mounted on the first substrate 701. Note that there is no particular limitation on the method of connecting a separately prepared driver circuit substrate, and a chip on glass (COG) method, a wire bonding method, or the like can be used.

The pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 included in the display device 700 include a plurality of transistors. As the plurality of transistors, any of the transistors that are the semiconductor devices of embodiments of the present invention can be used.

The display device 700 can include any of a variety of elements. The element includes, for example, at least one of a liquid crystal element, an electroluminescence (EL) element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element), an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor that emits light depending on current), an electron emitter, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a display element using micro electro mechanical systems (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), an interferometric modulator display (IMOD) element, a MEMS shutter display element, an optical-interference-type MEMS display element, an electrowetting element, a piezoelectric ceramic display, and a display element including a carbon nanotube. Other than the above elements, display media whose contrast, luminance, reflectivity, transmittance, or the like is changed may be included. Examples of display devices having EL elements include an EL display. Examples of display devices including electron emitters include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of display devices including liquid crystal elements include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). An example of a display device including electronic ink or electrophoretic elements is electronic paper. In the case of a transflective liquid crystal display or a reflective liquid crystal display, some of or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to include aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes, leading to lower power consumption.

As a display method in the display device 700, a progressive method, an interlace method, or the like can be employed. Furthermore, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, four pixels of the R pixel, the G pixel, the B pixel, and a W (white) pixel may be included. Alternatively, a color element may be composed of two colors among R, G, and B as in PenTile layout. The two colors may differ among color elements. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Furthermore, the size of a display region may be different depending on respective dots of the color components. Embodiments of the disclosed invention are not limited to a display device for color display; the disclosed invention can also be applied to a display device for monochrome display.

A coloring layer (also referred to as a color filter) may be used in order to obtain a full-color display device in which white light (W) for a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp) is used. As the coloring layer, red (R), green (G), blue (B), yellow (Y), or the like may be combined as appropriate, for example. With the use of the coloring layer, higher color reproducibility can be obtained than in the case without the coloring layer. In this case, by providing a region with the coloring layer and a region without the coloring layer, white light in the region without the coloring layer may be directly utilized for display. By partly providing the region without the coloring layer, a decrease in luminance due to the coloring layer can be suppressed, and 20% to 30% of power consumption can be reduced in some cases when an image is displayed brightly. Note that in the case where full-color display is performed using a self-luminous element such as an organic EL element or an inorganic EL element, elements may emit light of their respective colors R, G, B, Y, and W. By using a self-luminous element, power consumption can be further reduced as compared with the case of using the coloring layer in some cases.

In this embodiment, a structure including a liquid crystal element and an EL element as display elements is described with reference to FIG. 28 and FIG. 29. Note that FIG. 28 is a cross-sectional view along the dashed-dotted line Q-R shown in FIG. 27 and shows a structure including a liquid crystal element as a display element, whereas FIG. 29 is a cross-sectional view along the dashed-dotted line Q-R shown in FIG. 27 and shows a structure including an EL element as a display element.

Common portions between FIG. 28 and FIG. 29 are described first, and then different portions are described.

<Common Portions in Display Devices>

The display device 700 illustrated in FIG. 28 and FIG. 29 include a lead wiring portion 711, the pixel portion 702, the source driver circuit portion 704, and the FPC terminal portion 708. Note that the lead wiring portion 711 includes the signal line 710. The pixel portion 702 includes a transistor 750 and a capacitor 790. The source driver circuit portion 704 includes a transistor 752.

Any of the transistors described above can be used as the transistors 750 and 752.

The transistors used in this embodiment each include an oxide semiconductor film which is highly purified and in which formation of oxygen vacancy is suppressed. In the transistor, the current in an off state (off-state current) can be made small. Accordingly, an electrical signal such as an image signal can be held for a longer period, and a writing interval can be set longer in an on state. Accordingly, frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption.

Furthermore, any of the transistors described in Embodiment 1 can be used as the transistor in the pixel portion. Meanwhile, an L_(ov) transistor having high field-effect mobility is preferably used as the transistor in the driver circuit portion. Note that any of the transistors described in Embodiment 1 may be used in the driver circuit portion. Any of the transistors described in Embodiment 1 and the L_(ov) transistor can be separately fabricated by changing the pattern of the first gate electrode, the second gate electrode, the oxide semiconductor film, the source electrode, or the drain electrode, for example; thus, a switching transistor in the pixel portion and a driver transistor in the driver circuit portion can be formed over the same substrate. The use of any of the transistors described in Embodiment 1 as the transistor in the pixel portion can reduce parasitic capacitance, which leads to a reduction in time constant in switching operation of the pixel portion. Thus, a high-definition display device with a large panel size can be manufactured. That is, a semiconductor device formed using a silicon wafer or the like is not additionally needed as a driver circuit, by which the number of components of the semiconductor device can be reduced.

The capacitor 790 includes a dielectric between a pair of electrodes. Specifically, a conductive film which is formed using the same step as a conductive film functioning as a first gate electrode of the transistor 750 is used as one electrode of the capacitor 790, and a conductive film functioning as a source electrode or a drain electrode of the transistor 750 is used as the other electrode of the capacitor 790. Furthermore, an insulating film functioning as a gate insulating film of the transistor 750 is used as the dielectric between the pair of electrodes.

In FIG. 28 and FIG. 29, insulating films 764, 766, and 768, an oxide semiconductor film 767, and a planarization insulating film 770 are formed over the transistor 750, the transistor 752, and the capacitor 790.

The insulating films 764, 766, and 768 can be formed using materials and methods similar to those of the insulating films 114, 116, and 118 described in the above embodiment, respectively. The oxide semiconductor film 767 can be formed using a material and a method similar to those of the oxide semiconductor film 117 described in the above embodiment. The planarization insulating film 770 can be formed using a heat-resistant organic material, such as a polyimide resin, an acrylic resin, a polyimide amide resin, a benzocyclobutene resin, a polyamide resin, or an epoxy resin. Note that the planarization insulating film 770 may be formed by stacking a plurality of insulating films formed from these materials. Alternatively, a structure without the planarization insulating film 770 may be employed.

The signal line 710 is formed in the same steps as conductive films functioning as source and drain electrodes of the transistor 750 or 752. Note that the signal line 710 may be formed using a conductive film which is formed in different steps as a source electrode and a drain electrode of the transistor 750 or 752, for example, a conductive film functioning as a first gate electrode may be used. In the case where the signal line 710 is formed using a material including a copper element, signal delay or the like due to wiring resistance is reduced, which enables display on a large screen.

The FPC terminal portion 708 includes a connection electrode 760, an anisotropic conductive film 780, and the FPC 716. Note that the connection electrode 760 is formed in the same steps as the conductive films functioning as source and drain electrodes of the transistor 750 or 752. The connection electrode 760 is electrically connected to a terminal included in the FPC 716 through the anisotropic conductive film 780.

For example, a glass substrate can be used as the first substrate 701 and the second substrate 705. A flexible substrate may be used as the first substrate 701 and the second substrate 705. Examples of the flexible substrate include a plastic substrate.

A structure body 778 is provided between the first substrate 701 and the second substrate 705. The structure body 778 is a columnar spacer obtained by selective etching of an insulating film and provided to control the distance (cell gap) between the first substrate 701 and the second substrate 705. Note that a spherical spacer may be used as the structure body 778. Although the structure in which the structure body 778 is provided on the first substrate 701 side is described as an example in this embodiment, one embodiment of the present invention is not limited thereto. For example, a structure in which the structure body 778 is provided on the second substrate 705 side, or a structure in which both of the first substrate 701 and the second substrate 705 are provided with the structure body 778 may be employed.

Furthermore, a light-blocking film 738 functioning as a black matrix, a coloring film 736 functioning as a color filter, and an insulating film 734 in contact with the light-blocking film 738 and the coloring film 736 are provided on the second substrate 705 side.

<Structure Example of Display Device Using Liquid Crystal Element as Display Element>

The display device 700 in FIG. 28 includes a liquid crystal element 775. The liquid crystal element 775 includes a conductive film 772, a conductive film 774, and a liquid crystal layer 776. The conductive film 774 is provided on the second substrate 705 side and functions as a counter electrode. The display device 700 in FIG. 28 is capable of displaying an image in such a manner that transmission or non-transmission is controlled by change in the alignment state of the liquid crystal layer 776 depending on a voltage applied to the conductive film 772 and the conductive film 774.

The conductive film 772 is connected to the conductive film functioning as a source electrode or a drain electrode included in the transistor 750. The conductive film 772 is formed over the planarization insulating film 770 to function as a pixel electrode, that is, one electrode of the display element. The conductive film 772 functions as a reflective electrode. The display device 700 in FIG. 28 is what is called a reflective color liquid crystal display device in which external light is reflected by the conductive film 772 to display an image through the coloring film 736.

A conductive film that transmits visible light or a conductive film that reflects visible light can be used as the conductive film 772. For example, a material including one of indium (In), zinc (Zn), and tin (Sn) is preferably used for the conductive film that transmits visible light. For example, a material including aluminum or silver may be used for the conductive film that reflects visible light. In this embodiment, the conductive film that reflects visible light is used as the conductive film 772.

In the case where a conductive film which reflects visible light is used as the conductive film 772, the conductive film may have a stacked-layer structure. For example, a 100-nm-thick aluminum film is formed as the bottom layer, and a 30-nm-thick silver alloy film (e.g., an alloy film including silver, palladium, and copper) is formed as the top layer. Such a structure makes it possible to obtain the following effects.

(1) Adhesion between the base film and the conductive film 772 can be improved.

(2) The aluminum film and the silver alloy film can be collectively etched depending on a chemical solution.

(3) The conductive film 772 can have a favorable cross-sectional shape (e.g., a tapered shape).

The reason for (3) is as follows: the etching rate of the aluminum film with the chemical solution is lower than that of the silver alloy film, or etching of the aluminum film that is the bottom layer is developed faster than that of the silver alloy film because, when the aluminum film that is the bottom layer is exposed after the etching of the silver alloy film that is the top layer, electrons are extracted from metal that is less noble than the silver alloy film, that is, aluminum that is metal having a high ionization tendency, and thus etching of the silver alloy film is suppressed.

Note that projections and depressions are provided in part of the planarization insulating film 770 of the pixel portion 702 in the display device 700 in FIG. 28. The projections and depressions can be formed in such a manner that the planarization insulating film 770 is formed using an organic resin film or the like, and projections and depressions are formed on the surface of the organic resin film. The conductive film 772 functioning as a reflective electrode is formed along the projections and depressions. Therefore, when external light is incident on the conductive film 772, the light is reflected diffusely at the surface of the conductive film 772, whereby visibility can be improved.

Note that the display device 700 in FIG. 28 is a reflective color liquid crystal display device given as an example, but a display type is not limited thereto. For example, a transmissive color liquid crystal display device in which a conductive film that transmits visible light is used as the conductive film 772 may be used. In the case of a transmissive color liquid crystal display device, projections and depressions are not necessarily provided on the planarization insulating film 770.

Although not illustrated in FIG. 28, an alignment film may be provided on a side of the conductive film 772 in contact with the liquid crystal layer 776 and on a side of the conductive film 774 in contact with the liquid crystal layer 776. Although not illustrated in FIG. 28, an optical member (an optical substrate) and the like such as a polarizing member, a retardation member, or an anti-reflection member may be provided as appropriate. For example, circular polarization may be employed by using a polarizing substrate and a retardation substrate. In addition, a backlight, a sidelight, or the like may be used as a light source.

In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer-dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric field mode, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral material has a short response time, and has optical isotropy, which makes the alignment process unneeded and viewing angle dependence small. An alignment film does not need to be provided and rubbing treatment is thus not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced.

In the case where a liquid crystal element is used as the display element, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used.

Furthermore, a normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode may also be used. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, or the like can be employed.

Although image data is generally rewritten at 60 Hz or the like in a display device including a liquid crystal element, a transistor including an oxide semiconductor has an extremely low off-state current; thus, the write frequency can be further lowered. For example, the write frequency can be lowered to 15 Hz, to further 1 Hz. When the write frequency is low, power consumption can be lowered, for example.

<Display Device Using Light-Emitting Element as Display Element>

The display device 700 illustrated in FIG. 29 includes a light-emitting element 782. The light-emitting element 782 includes a conductive film 784, an EL layer 786, and a conductive film 788. The display device 700 illustrated in FIG. 29 is capable of displaying an image by light emission from the EL layer 786 included in the light-emitting element 782.

The conductive film 784 is connected to the conductive film functioning as a source electrode or a drain electrode included in the transistor 750. The conductive film 784 is formed over the planarization insulating film 770 to function as a pixel electrode, that is, one electrode of the display element. A conductive film which transmits visible light or a conductive film which reflects visible light can be used as the conductive film 784. The conductive film which transmits visible light can be formed using a material including one of indium (In), zinc (Zn), and tin (Sn), for example. The conductive film which reflects visible light can be formed using a material including aluminum or silver, for example.

In the display device 700 in FIG. 29, an insulating film 730 is provided over the planarization insulating film 770 and the conductive film 784. The insulating film 730 covers part of the conductive film 784. Note that the light-emitting element 782 has a top emission structure. Therefore, the conductive film 788 has a light-transmitting property and transmits light emitted from the EL layer 786. Although the top-emission structure is described as an example in this embodiment, one embodiment of the present invention is not limited thereto. A bottom-emission structure in which light is emitted to the conductive film 784 side, or a dual-emission structure in which light is emitted to both the conductive film 784 side and the conductive film 788 side may be employed.

The coloring film 736 is provided to overlap with the light-emitting element 782, and the light-blocking film 738 is provided to overlap with the insulating film 730 and to be included in the lead wiring portion 711 and in the source driver circuit portion 704. The coloring film 736 and the light-blocking film 738 are covered with the insulating film 734. A space between the light-emitting element 782 and the insulating film 734 is filled with a sealing film 732. Although a structure with the coloring film 736 is described as the display device 700 in FIG. 29, the structure is not limited thereto. In the case where the EL layer 786 is formed by a separate coloring method, the coloring film 736 is not necessarily provided.

The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments.

Embodiment 4 Example of Problem Solvable by One Embodiment of the Present Invention

A problem which can be solved by one embodiment of the present invention is described with reference to FIGS. 32A to 32C and FIGS. 33A to 33D. Specifically, the problem is that when driving a liquid crystal element with AC voltage in order to prevent deterioration of the liquid crystal element, flicker is caused by feedthrough, so that a user of a display device feels eye fatigue.

<<Structure of Display Portion and Pixel>>

FIG. 32A is a block diagram of a display portion in a display device, FIG. 32B is an equivalent circuit diagram of a pixel circuit included in a pixel of the display portion, and FIG. 32C is a schematic view of polarized light transmittance versus voltage characteristics of a liquid crystal element.

FIGS. 33A to 33D are timing charts and a schematic view showing operations of a pixel circuit of a pixel portion in a display device.

<<Display Device>>

A display portion 930 is part of a display device. The display portion 930 includes a pixel portion 931 (see FIG. 32A).

The pixel portion 931 includes a plurality of pixels 931 p, a plurality of signal lines S (S1 to Sx) supplying first driving signals (also referred to as S signals) to the pixels 931 p column by column, and a plurality of scan lines G (G1 to Gy) supplying second driving signals (also referred to as G signals) to the pixels 931 p row by row.

Each of the plurality of pixels 931 p is connected to at least one of the scan lines G and at least one of the signal lines S.

Note that signals for selecting the pixels 931 p row by row may be used as the G signals, and the S signals including grayscale information on an image and the like may be supplied to the pixels 931 p selected in accordance with the G signals.

A G driver circuit 932 can control input of the G signals to the scan lines G. An S driver circuit 933 can control input of the S signals to the signal lines S.

The pixel 931 p described here includes a liquid crystal element 935LC and a pixel circuit 934 including the liquid crystal element 935LC (see FIG. 32B).

The pixel circuit 934 includes a transistor 934 t which can control supply of the S signal to the liquid crystal element 935LC.

A gate of the transistor 934 t is electrically connected to any one of the scan lines G. A first electrode (one of a source and a drain) of the transistor 934 t is electrically connected to any one of the signal lines S. A second electrode (the other of the source and the drain) of the transistor 934 t is electrically connected to a first electrode of the liquid crystal element 935LC. A second electrode of the liquid crystal element 935LC is electrically connected to a common line C.

Note that the transistor 934 t has a parasitic capacitance 934 tc. The parasitic capacitance 934 tc includes capacitance between the gate electrode and the source or the drain and capacitance between the gate electrode and a channel.

The pixel circuit 934 may further include a capacitor 934 c which can hold voltage between the first electrode and the second electrode of the liquid crystal element 935LC.

The G signal is input to the gate electrode of the transistor 934 t, and the transistor 934 t can control, as a switching element, input of the S signal to the liquid crystal element 935LC.

The liquid crystal element 935LC includes the first electrode, the second electrode, and a liquid crystal layer including a liquid crystal material to which the voltage between the first electrode and the second electrode is applied.

The polarized light transmittance of the liquid crystal element 935LC is dependent on the alignment state of liquid crystal molecules included in the liquid crystal layer. The alignment state in the liquid crystal layer can be controlled by voltage applied between the first electrode and the second electrode of the liquid crystal element 935LC. Accordingly, when a voltage corresponding to the S signal including grayscale information on an image is applied between the first electrode and the second electrode of the liquid crystal element 935LC, the polarized light transmittance of the liquid crystal element 935LC can be controlled to correspond to the grayscale information.

<<Characteristics of Liquid Crystal Element>>

FIG. 32C schematically shows the relation between the polarized light transmittance of a normally-white liquid crystal element and voltage applied to the liquid crystal element. The vertical axis represents the polarized light transmittance, and the horizontal axis represents the potential of a first electrode of the liquid crystal element with respect to a second electrode.

The polarized light transmittance of the normally-white liquid crystal element is high when the voltage between the first electrode and the second electrode is zero, and is decreased as the voltage is increased. A transmissive liquid crystal display device including such a liquid crystal element can display white when the voltage of the S signal is close to zero and can display black when the voltage of the S signal is high.

It is known that a liquid crystal element deteriorates when DC voltage is kept being applied to a liquid crystal layer of the liquid crystal element for a long time. To prevent such deterioration, a liquid crystal element needs to be driven with AC voltage.

To keep the polarized light transmittance of the liquid crystal element at a transmittance Ta, the liquid crystal element is driven while the polarity of the first electrode with respect to the second electrode is switched between positive (potential +Va) and negative (potential −Va). In other words, the liquid crystal element is driven with AC voltage with constant amplitude.

<<Method for Driving Liquid Crystal Element>>

A method for driving the liquid crystal element 935LC of the pixel circuit 934 with AC voltage is described.

FIG. 33A is a timing chart of a potential Vg of the G signal input to the gate electrode of the transistor 934 t of the pixel circuit 934 and a potential Vsa of the S signal input to the first electrode of the transistor 934 t. FIG. 33B is a timing chart of a potential Vs output from the second electrode of the transistor 934 t. Note that the potential Vs is to be input to the first electrode of the liquid crystal element 935LC.

A secondary image signal is input to the S driver circuit 933, and the S driver circuit 933 outputs the S signal. The S signal may have the same amplitude as the secondary image signal. Note that the secondary image signal is generated from a primary image signal and can have an amplitude corresponding to a difference between the potential of the primary image signal and a reference potential Vsc. The secondary image signal is a signal whose polarity is inverted every frame.

A period between when one scan line G provided in the pixel portion 931 is selected and when the same scan line G is selected again is referred to as one frame. Accordingly, the G signal is input to the gate electrode of the transistor 934 t every frame (see FIG. 33A). The potential Vg is VgH when the G signal is high, and is VgL when the G signal is low.

<<Generation of Feedthrough>>

The potential Vs of the second electrode of the transistor 934 t is affected by the potential Vg of the gate electrode. Thus, the potential Vs is different from the potential Vsa (which is equal to the potential of the secondary image signal) of the S signal input to the first electrode of the transistor 934 t. Described below is an example where there are a period during which the potential of the secondary image signal is higher than the reference potential Vsc by Vsa1 and a period during which the potential of the secondary image signal is lower than the reference potential Vsc by Vsa2. The secondary image signal is input to the S driver circuit, and the S driver circuit generates the S signal having the same potential as the secondary image signal.

In a first frame F1, when a difference between the potential Vg of the gate electrode and the potential Vsa of the S signal exceeds the threshold voltage Vth of the transistor 934 t, the transistor 934 t is turned on and the potential Vs is raised to the potential Vsa.

Then, when the difference between the potential Vg of the gate electrode and the potential Vsa of the S signal becomes smaller than the threshold voltage Vth, the transistor 934 t is turned off and the potential Vs of the second electrode of the transistor 934 t is lowered from the potential Vsa of the S signal by ΔV1. The amount by which the potential Vs of the second electrode of the transistor 934 t is lowered from the potential Vsa of the S signal is referred to as a first feedthrough ΔV1.

In a second frame F2, the secondary image signal whose polarity has been inverted is input to the S driver circuit 933. When the difference between the potential Vg and the potential Vsa of the S signal exceeds the threshold voltage Vth of the transistor 934 t, the transistor 934 t is turned on and the potential Vs of the second electrode of the transistor 934 t is lowered to the potential Vsa of the S signal.

Then, when the difference between the potential Vg of the gate electrode and the potential Vsa of the S signal becomes smaller than the threshold voltage Vth, the transistor 934 t is turned off and the potential Vs of the second electrode of the transistor 934 t is lowered from the potential Vsa by ΔV2. The amount by which the potential Vs of the second electrode of the transistor 934 t is lowered from the potential Vsa is referred to as a second feedthrough ΔV2.

<<Amount of Feedthrough>>

Feedthrough is caused by capacitive coupling due to the parasitic capacitance 934 tc of the transistor 934 t (see FIG. 32B). The amount ΔV of the feedthrough can be estimated from a formula (3) below. Note that in the formula, Vg_(LH) represents the amplitude (a difference between VgH and VgL) of the potential of the gate electrode, CL represents the capacitance of the liquid crystal element 935LC, Cs represents the capacitance of the capacitor 934 c, and Cdg represents the parasitic capacitance 934 tc of the transistor 934 t.

$\begin{matrix} {{\Delta\; V} = {{Vg}_{LH} \times \frac{Cdg}{{CL} + {Cs} + {Cdg}}}} & (3) \end{matrix}$ <<Asymmetry of Amount of Feedthrough>>

The G signal input to the gate electrode of the transistor 934 t does not have a complete rectangular waveform. For example, in the case where the G signal has a trapezoidal waveform whose upper base is shorter than the lower base, the transistor 934 t is in an on state for a longer time when the potential Vs of the input S signal is low than when the potential Vs of the input S signal is high. Therefore, since a value of Cdg is changed by the effect of a channel capacitance or the like, the second feedthrough ΔV2 is larger than the first feedthrough ΔV1, that is, the first feedthrough ΔV1 and the second feedthrough ΔV2 are asymmetric.

Note that even in the case where the first feedthrough ΔV1 and the second feedthrough ΔV2 are asymmetric, the amounts of feedthrough can be estimated on the basis of the potential of the input S signal when the waveform of the G signal is kept constant.

<<Effect of Feedthrough on Polarized Light Transmittance of Liquid Crystal Element>>

When a liquid crystal element is driven with AC voltage with constant amplitude, the polarized light transmittance of the liquid crystal element is kept constant.

However, the polarized light transmittance of the liquid crystal element cannot be kept constant when feedthrough is caused in the pixel circuit.

The action of the first feedthrough ΔV1 reduces |Vs−Vsc|, a difference between the potential Vs of the second electrode of the transistor 934 t and the reference potential Vsc. The action of the second feedthrough ΔV2 increases |Vs−Vsc|, a difference between the potential Vs of the second electrode of the transistor 934 t and the reference potential Vsc (see FIG. 33C).

Thus, when the liquid crystal element 935LC is a normally-white liquid crystal element, the polarized light transmittance of the liquid crystal element 935LC is increased from the transmittance Ta by ΔT1 in the first frame F1 and is decreased from the transmittance Ta by ΔT2 in the second frame F2 (see FIG. 33D). The common potential Vcom is a common potential input to the second electrode of the liquid crystal element 935LC and is equal to the reference potential Vsc.

As a result, the polarized light transmittance of the liquid crystal element 935LC in the display portion of the display device is changed (specifically, the amount of change corresponds to the sum of ΔT1 and ΔT2), so that flicker is caused.

In general, flicker is made invisible by adjusting a common potential. A variation in characteristics or capacitance in a plane occurs might result in an observation of flicker. To suppress generation of flicker, it can be considered that storage capacitance is increased, for example; however, an aperture ratio is affected in that case. Flicker is less likely to occur in the case where feedthrough is made small as in the present invention.

<<Eye Fatigue>>

Eye Fatigue Includes Nervous Asthenopia.

Nervous asthenopia is caused when a user keeps looking at continuous or blinking light emitted from a display portion for a long time. This is because the brightness stimulates and fatigues the retina or nerve of the eye or the brain. Frequent blinking of fluorescent light or a display portion of a conventional display device, which is called flicker, causes nervous asthenopia.

As described above, there is a problem in that when AC voltage is applied to a liquid crystal element so that the liquid crystal element is driven while being prevented from deteriorating, flicker is caused by feedthrough, so that a user of a display device feels eye fatigue.

The transistors in Embodiment 1 have small parasitic capacitance and high field-effect mobility. The use of such a transistor having high performance and small parasitic capacitance enables a reduction in various problems caused by feedthrough which are described in this embodiment.

Embodiment 5

In this embodiment, a display device that includes a semiconductor device of one embodiment of the present invention will be described with reference to FIGS. 30A to 30C.

The display device illustrated in FIG. 30A includes a region including pixels of display elements (hereinafter the region is referred to as a pixel portion 502), a circuit portion being provided outside the pixel portion 502 and including a circuit for driving the pixels (hereinafter the portion is referred to as a driver circuit portion 504), circuits each having a function of protecting an element (hereinafter the circuits are referred to as protection circuits 506), and a terminal portion 507. Note that the protection circuits 506 are not necessarily provided.

Part or the whole of the driver circuit portion 504 is preferably formed over a substrate over which the pixel portion 502 is formed, in which case the number of components and the number of terminals can be reduced. When part or the whole of the driver circuit portion 504 is not formed over the substrate over which the pixel portion 502 is formed, the part or the whole of the driver circuit portion 504 can be mounted by COG or tape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (hereinafter, such circuits are referred to as pixel circuits 501). The driver circuit portion 504 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (hereinafter, the circuit is referred to as a gate driver 504 a) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (hereinafter, the circuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gate driver 504 a receives a signal for driving the shift register through the terminal portion 507 and outputs a signal. For example, the gate driver 504 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The gate driver 504 a has a function of controlling the potentials of wirings supplied with scan signals (hereinafter, such wirings are referred to as scan lines GL_1 to GL_X). Note that a plurality of gate drivers 504 a may be provided to control the scan lines GL_1 to GL_X separately. Alternatively, the gate driver 504 a has a function of supplying an initialization signal. Without being limited thereto, the gate driver 504 a can supply another signal.

The source driver 504 b includes a shift register or the like. The source driver 504 b receives a signal (image signal) from which a data signal is derived, as well as a signal for driving the shift register, through the terminal portion 507. The source driver 504 b has a function of generating a data signal to be written to the pixel circuit 501 which is based on the image signal. In addition, the source driver 504 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the source driver 504 b has a function of controlling the potentials of wirings supplied with data signals (hereinafter such wirings are referred to as data lines DL_1 to DL_Y). Alternatively, the source driver 504 b has a function of supplying an initialization signal. Without being limited thereto, the source driver 504 b can supply another signal.

The source driver 504 b includes a plurality of analog switches, for example. The source driver 504 b can output, as the data signals, signals obtained by time-dividing the image signal by sequentially turning on the plurality of analog switches. The source driver 504 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality of pixel circuits 501 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal to and in each of the plurality of pixel circuits 501 are controlled by the gate driver 504 a. For example, to the pixel circuit 501 in the m-th row and the n-th column (m is a natural number of less than or equal to X, and n is a natural number of less than or equal to 1), a pulse signal is input from the gate driver 504 a through the scan line GL_m, and a data signal is input from the source driver 504 b through the data line DL_n in accordance with the potential of the scan line GL_m.

The protection circuit 506 illustrated in FIG. 30A is connected to, for example, the scan line GL between the gate driver 504 a and the pixel circuit 501. Alternatively, the protection circuit 506 is connected to the data line DL between the source driver 504 b and the pixel circuit 501. Alternatively, the protection circuit 506 can be connected to a wiring between the gate driver 504 a and the terminal portion 507. Alternatively, the protection circuit 506 can be connected to a wiring between the source driver 504 b and the terminal portion 507. Note that the terminal portion 507 means a portion having terminals for inputting power, control signals, and image signals to the display device from external circuits.

The protection circuit 506 is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit.

As illustrated in FIG. 30A, the protection circuits 506 are provided for the pixel portion 502 and the driver circuit portion 504, so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved. Note that the configuration of the protection circuits 506 is not limited to that, and for example, the protection circuit 506 may be configured to be connected to the gate driver 504 a or the protection circuit 506 may be configured to be connected to the source driver 504 b. Alternatively, the protection circuit 506 may be configured to be connected to the terminal portion 507.

In FIG. 30A, an example in which the driver circuit portion 504 includes the gate driver 504 a and the source driver 504 b is shown; however, the structure is not limited thereto. For example, only the gate driver 504 a may be formed and a separately prepared substrate where a source driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 30A can have the structure illustrated in FIG. 30B, for example.

The pixel circuit 501 illustrated in FIG. 30B includes a liquid crystal element 570, a transistor 550, and a capacitor 560. As the transistor 550, any of the transistors described in the above embodiment can be used.

The potential of one of a pair of electrodes of the liquid crystal element 570 is set in accordance with the specifications of the pixel circuit 501 as appropriate. The alignment state of the liquid crystal element 570 depends on written data. A common potential may be supplied to one of the pair of electrodes of the liquid crystal element 570 included in each of the plurality of pixel circuits 501. Furthermore, the potential supplied to one of the pair of electrodes of the liquid crystal element 570 in the pixel circuit 501 in one row may be different from the potential supplied to one of the pair of electrodes of the liquid crystal element 570 in the pixel circuit 501 in another row.

As examples of a driving method of the display device including the liquid crystal element 570, any of the following modes can be given: a TN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode, and the like. Other examples of the driving method of the display device include an electrically controlled birefringence (ECB) mode, a polymer-dispersed liquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC) mode, and a guest-host mode. Note that the present invention is not limited to these examples, and various liquid crystal elements and driving methods can be applied to the liquid crystal element and the driving method thereof.

In the pixel circuit 501 in the m-th row and the n-th column, one of a source electrode and a drain electrode of the transistor 550 is electrically connected to the data line DL_n, and the other is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. A gate electrode of the transistor 550 is electrically connected to the scan line GL_m. The transistor 550 has a function of controlling whether to write a data signal by being turned on or off.

One of a pair of electrodes of the capacitor 560 is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL), and the other is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. The potential of the potential supply line VL is set in accordance with the specifications of the pixel circuit 501 as appropriate. The capacitor 560 functions as a storage capacitor for storing written data.

For example, in the display device including the pixel circuit 501 in FIG. 30B, the pixel circuits 501 are sequentially selected row by row by the gate driver 504 a illustrated in FIG. 30A, whereby the transistors 550 are turned on and a data signal is written.

When the transistors 550 are turned off, the pixel circuits 501 in which the data has been written are brought into a holding state. This operation is sequentially performed row by row; thus, an image can be displayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 30A can have the structure illustrated in FIG. 30C, for example.

The pixel circuit 501 illustrated in FIG. 30C includes transistors 552 and 554, a capacitor 562, and a light-emitting element 572. Any of the transistors described in the above embodiment can be used as one or both of the transistors 552 and 554.

One of a source electrode and a drain electrode of the transistor 552 is electrically connected to a wiring to which a data signal is supplied (hereinafter referred to as a data line DL_n). A gate electrode of the transistor 552 is electrically connected to a wiring to which a gate signal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 552 has a function of controlling whether to write a data signal by being turned on or off.

One of a pair of electrodes of the capacitor 562 is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for storing written data.

One of a source electrode and a drain electrode of the transistor 554 is electrically connected to the potential supply line VL_a. Furthermore, a gate electrode of the transistor 554 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.

One of an anode and a cathode of the light-emitting element 572 is electrically connected to a potential supply line VL_b, and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element (also referred to as an organic EL element) can be used, for example. Note that the light-emitting element 572 is not limited to an organic EL element; an inorganic EL element including an inorganic material may be used.

A high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other.

For example, in the display device including the pixel circuit 501 in FIG. 30C, the pixel circuits 501 are sequentially selected row by row by the gate driver 504 a illustrated in FIG. 30A, whereby the transistors 552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 501 in which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor 554 is controlled in accordance with the potential of the written data signal. The light-emitting element 572 emits light with luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image can be displayed.

Parasitic capacitance C_(GD) occurs in a region 580 in FIGS. 30B and 30C where the GL_m line and the DL_n line overlap with each other. Similarly to the parasitic capacitance of a transistor, large parasitic capacitance C_(GD) causes long rise time, in which case an increase in the panel size of the display device becomes difficult. Accordingly, it is preferable to reduce the capacitance between the GL_m line and the DL_n line.

In general, the insulating films 106 and 107 which are formed in the same steps as the first gate insulating film are provided between the GL_m line and the DL_n line. Inserting an insulating film into the insulating films 106 and 107 between the GL_m line and the DL_n line to obtain an insulating film with a large total thickness is preferable because parasitic capacitance of a transistor can be reduced.

FIG. 31A is a top view showing the vicinity of the region 580 in FIGS. 30B and 30C. FIG. 31B is a cross-sectional view taken along a line Z1-Z2 in FIG. 31A. For example, as shown in FIGS. 31A and 31B, the insulating film between the GL_m line and the DL_n line in the case where a stacked structure of the insulating film 106, the insulating film 107, and an insulating film 121 is provided between the GL_m line and the DL_n line has a larger thickness than that in the case where only the insulating film 106 and the insulating film 107 are provided between the GL_m line and the DL_n line; thus, the parasitic capacitance C_(GD) between the GL_m line and the DL_n line is decreased. To decrease the parasitic capacitance C_(GD), a material with a low dielectric constant is preferably used for the insulating film 121.

As the insulating film 121, a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, an aluminum oxynitride film, an aluminum nitride oxide film, an aluminum nitride film, or the like can be used.

The insulating film 121 can be formed by a sputtering method, a CVD method (including a thermal CVD method, an MOCVD method, a PECVD method, and the like), an MBE method, an ALD method, a PLD method, or the like. In particular, it is preferable that the insulating film be formed by a CVD method, further preferably a plasma CVD method because coverage can be further improved. It is preferable to use a thermal CVD method, an MOCVD method, or an ALD method in order to reduce plasma damage.

The insulating film 121 can be formed using a silicon carbonitride film (SiCN film). Alternatively, undoped silicate glass (USG), boron phosphorus silicate glass (BPSG), borosilicate glass (BSG), or the like can be used. USG, BPSG, and the like may be formed by an atmospheric pressure CVD method. Alternatively, hydrogen silsesquioxane (HSQ) or the like may be applied by a coating method.

Note that the insulating film 121 may have a stacked-layer structure of two or more layers. For example, USG may be stacked over an insulating film including nitrogen and silicon.

The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments.

Embodiment 6

In this embodiment, a display module and electronic devices that include a semiconductor device of one embodiment of the present invention will be described with reference to FIG. 34, FIGS. 35A to 35G, and FIGS. 36A to 36G.

In a display module 8000 illustrated in FIG. 34, a touch panel 8004 connected to an FPC 8003, a display panel 8006 connected to an FPC 8005, a backlight 8007, a frame 8009, a printed board 8010, and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002.

The semiconductor device of one embodiment of the present invention can be used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch panel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitive touch panel and can be formed to overlap with the display panel 8006. A counter substrate (sealing substrate) of the display panel 8006 can have a touch panel function. A photosensor may be provided in each pixel of the display panel 8006 to form an optical touch panel.

The backlight 8007 includes a light source 8008. Note that although a structure in which the light sources 8008 are provided over the backlight 8007 is illustrated in FIG. 34, one embodiment of the present invention is not limited to this structure. For example, a structure in which the light source 8008 is provided at an end portion of the backlight 8007 and a light diffusion plate is further provided may be employed. Note that the backlight 8007 need not be provided in the case where a self-luminous light-emitting element such as an organic EL element is used or in the case where a reflective panel or the like is employed.

The frame 8009 protects the display panel 8006 and also functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board 8010. The frame 8009 may function as a radiator plate.

The printed board 8010 is provided with a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or a power source using the battery 8011 provided separately may be used. The battery 8011 can be omitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 35A to 35G illustrate electronic devices. These electronic devices can include a housing 9000, a display portion 9001, a speaker 9003, operation keys 9005 (including a power switch or an operation switch), a connection terminal 9006, a sensor 9007 (a sensor having a function of measuring or sensing force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic devices illustrated in FIGS. 35A to 35G can have a variety of functions, for example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion, and the like. Note that functions that can be provided for the electronic devices illustrated in FIGS. 35A to 35G are not limited to those described above, and the electronic devices can have a variety of functions. Although not illustrated in FIGS. 35A to 35G, the electronic device may include a plurality of display portions. Furthermore, the electronic device may be provided with a camera and the like and have a function of shooting a still image, a function of shooting a moving image, a function of storing a shot image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying a shot image on the display portion, or the like.

The electronic devices illustrated in FIGS. 35A to 35G are described in detail below.

FIG. 35A is a perspective view illustrating a portable information terminal 9100. The display portion 9001 of the portable information terminal 9100 is flexible. Therefore, the display portion 9001 can be incorporated along a bent surface of a bent housing 9000. Furthermore, the display portion 9001 includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, by touching an icon displayed on the display portion 9001, an application can be started.

FIG. 35B is a perspective view illustrating a portable information terminal 9101. The portable information terminal 9101 function as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal 9101 can be used as a smartphone. Note that although the speaker 9003, the connection terminal 9006, the sensor 9007, and the like of the portable information terminal 9101 are not illustrated in FIG. 35B, they can be provided in the same positions as the portable information terminal 9100 in FIG. 35A. The portable information terminal 9101 can display characters and image information on its plurality of surfaces. For example, three operation buttons 9050 (also referred to as operation icons, or simply, icons) can be displayed on one surface of the display portion 9001. Furthermore, information 9051 indicated by dashed rectangles can be displayed on another surface of the display portion 9001. Examples of the information 9051 include display indicating reception of an incoming email, social networking service (SNS) message, and call; the title and sender of an email and SNS massage; the date; the time; remaining battery; and the reception strength of an antenna. Alternatively, the operation buttons 9050 or the like may be displayed in place of the information 9051.

FIG. 35C is a perspective view illustrating a portable information terminal 9102. The portable information terminal 9102 has a function of displaying information, for example, on three or more sides of the display portion 9001. Here, information 9052, information 9053, and information 9054 are displayed on different sides. For example, a user of the portable information terminal 9102 can see the display (here, the information 9053) with the portable information terminal 9102 put in a breast pocket of his/her clothes. Specifically, a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal 9102. Thus, the user can see the display without taking out the portable information terminal 9102 from the pocket and decide whether to answer the call.

FIG. 35D is a perspective view illustrating a wrist-watch-type portable information terminal 9200. The portable information terminal 9200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, reading and editing texts, music reproduction, Internet communication, and a computer game. The display surface of the display portion 9001 is bent, and images can be displayed on the bent display surface. The portable information terminal 9200 can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the portable information terminal 9200 and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. Moreover, the portable information terminal 9200 includes the connection terminal 9006, and data can be directly transmitted to and received from another information terminal via a connector. Charging through the connection terminal 9006 is possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal 9006.

FIGS. 35E, 35F, and 35G are perspective views each illustrating a foldable portable information terminal 9201. FIG. 35E is a perspective view illustrating the portable information terminal 9201 that is opened, FIG. 35F is a perspective view illustrating the portable information terminal 9201 that is being opened or being folded, and FIG. 35G is a perspective view illustrating the portable information terminal 9201 that is folded. The portable information terminal 9201 is highly portable when folded. When the portable information terminal 9201 is opened, a seamless large display region is highly browsable. The display portion 9001 of the portable information terminal 9201 is supported by three housings 9000 joined together by hinges 9055. By folding the portable information terminal 9201 at a connection portion between two housings 9000 with the hinges 9055, the portable information terminal 9201 can be reversibly changed in shape from an opened state to a folded state. For example, the portable information terminal 9201 can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm.

FIGS. 36A to 36G illustrate electronic devices. These electronic devices can include a housing 5000, a display portion 5001, a speaker 5003, an LED lamp 5004, operation keys 5005 (including a power switch or an operation switch), a connection terminal 5006, a sensor 5007 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 5008, and the like.

FIG. 36A illustrates a mobile computer, which can include a switch 5009, an infrared port 5010, and the like in addition to the above components. FIG. 36B illustrates a portable image reproducing device (e.g., a DVD player), which is provided with a memory medium and can include a second display portion 5002, a memory medium reading portion 5011, and the like in addition to the above components. FIG. 36C illustrates a goggle-type display, which can include the second display portion 5002, a support 5012, an earphone 5013, and the like in addition to the above components. FIG. 36D illustrates a portable game machine, which can include the memory medium reading portion 5011 and the like in addition to the above components. FIG. 36E illustrates a digital camera, which has a television reception function and can include an antenna 5014, a shutter button 5015, an image receiving portion 5016, and the like in addition to the above components. FIG. 36F illustrates a portable game machine, which can include the second display portion 5002, the memory medium reading portion 5011, and the like in addition to the above components. FIG. 36G illustrates a portable television receiver that can include a charger 5017 capable of transmitting and receiving signals, and the like in addition to the above components.

The electronic devices illustrated in FIGS. 36A to 36G can have a variety of functions, such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on a display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading a program or data stored in a recording medium and displaying the program or data on a display portion. Furthermore, the electronic device including a plurality of display portions can have a function of displaying image information mainly on one display portion while displaying text information on another display portion, a function of displaying a three-dimensional image by displaying images where parallax is utilized on a plurality of display portions, or the like. Furthermore, the electronic device including an image receiving portion can have a function of photographing a still image, a function of photographing a moving image, a function of automatically or manually correcting a photographed image, a function of storing a photographed image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying a photographed image on the display portion, or the like. Note that the electronic devices shown in FIGS. 36A to 36G can have a variety of functions, not limited to the above functions.

Electronic devices described in this embodiment are characterized by having a display portion for displaying some sort of information. The display device described in Embodiment 5 can be applied to the display portion.

The electronic devices described in this embodiment each include the display portion for displaying some sort of data. Note that the semiconductor device of one embodiment of the present invention can also be used for an electronic device that does not have a display portion.

The structure described in this embodiment can be used in appropriate combination with any of the structures described in the other embodiments.

Embodiment 7 Memory Device

An example of a semiconductor device (memory device) which includes the transistor of one embodiment of the present invention, which can retain stored data even when not powered, and which has an unlimited number of write cycles is shown in FIGS. 40A to 40C. Note that FIG. 40B is a circuit diagram of the structure in FIG. 40A.

The semiconductor device illustrated in FIGS. 40A and 40B includes a transistor 3200 using a first semiconductor material, a transistor 3300 using a second semiconductor material, and a capacitor 3400. Note that any of the transistors described in Embodiment 1 can be used as the transistor 3300.

The transistor 3300 is a transistor in which a channel is formed in a semiconductor layer including an oxide semiconductor. Since the off-state current of the transistor 3300 is small, stored data can be retained for a long period. In other words, power consumption can be sufficiently reduced because a semiconductor memory device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided.

In FIG. 40B, a first wiring 3001 is electrically connected to a source electrode of the transistor 3200. A second wiring 3002 is electrically connected to a drain electrode of the transistor 3200. A third wiring 3003 is electrically connected to one of a source electrode and a drain electrode of the transistor 3300. A fourth wiring 3004 is electrically connected to a gate electrode of the transistor 3300. A gate electrode of the transistor 3200 and the other of the source electrode and the drain electrode of the transistor 3300 are electrically connected to the one electrode of the capacitor 3400. A fifth wiring 3005 is electrically connected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 40A has a feature that the potential of the gate electrode of the transistor 3200 can be retained, and thus enables writing, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of the fourth wiring 3004 is set to a potential at which the transistor 3300 is turned on, so that the transistor 3300 is turned on. Accordingly, the potential of the third wiring 3003 is supplied to the gate electrode of the transistor 3200 and the capacitor 3400. That is, a predetermined charge is supplied to the gate electrode of the transistor 3200 (writing). Here, one of two kinds of charges providing different potential levels (hereinafter referred to as a low-level charge and a high-level charge) is supplied. After that, the potential of the fourth wiring 3004 is set to a potential at which the transistor 3300 is turned off, so that the transistor 3300 is turned off Thus, the charge supplied to the gate electrode of the transistor 3200 is held (retaining).

Since the off-state current of the transistor 3300 is extremely small, the charge of the gate electrode of the transistor 3200 is retained for a long time.

Next, reading of data is described. An appropriate potential (a reading potential) is supplied to the fifth wiring 3005 while a predetermined potential (a constant potential) is supplied to the first wiring 3001, whereby the potential of the second wiring 3002 varies depending on the amount of charge retained in the gate electrode of the transistor 3200. This is because in the case of using an n-channel transistor as the transistor 3200, an apparent threshold voltage V_(th) _(_) _(H) at the time when the high-level charge is given to the gate electrode of the transistor 3200 is lower than an apparent threshold voltage V_(th) _(_) _(L) at the time when the low-level charge is given to the gate electrode of the transistor 3200. Here, an apparent threshold voltage refers to the potential of the fifth wiring 3005 which is needed to turn on the transistor 3200. Thus, the potential of the fifth wiring 3005 is set to a potential V₀ which is between V_(th) _(_) _(H) and V_(th) _(_) _(L), whereby charge supplied to the gate electrode of the transistor 3200 can be determined. For example, in the case where the high-level charge is supplied to the gate electrode of the transistor 3200 in writing and the potential of the fifth wiring 3005 is V₀ (>V_(th) _(_) _(H)), the transistor 3200 is turned on. On the other hand, in the case where the low-level charge is supplied to the gate electrode of the transistor 3200 in writing, even when the potential of the fifth wiring 3005 is V₀ (<V_(th) _(_) _(L)), the transistor 3200 remains off Thus, the data retained in the gate electrode of the transistor 3200 can be read by determining the potential of the second wiring 3002.

Note that in the case where memory cells are arrayed, it is necessary that data of a desired memory cell is read. For example, the fifth wiring 3005 of memory cells from which data is not read may be supplied with a potential at which the transistor 3200 is turned off regardless of the state of the gate electrode, that is, a potential lower than V_(th) _(_) _(H), whereby only data of a desired memory cell can be read. Alternatively, the fifth wiring 3005 of the memory cells from which data is not read may be supplied with a potential at which the transistor 3200 is turned on regardless of the state of the gate electrode, that is, a potential higher than V_(th) _(_) _(L), whereby only data of a desired memory cell can be read.

The semiconductor device illustrated in FIG. 40C is different from the semiconductor device illustrated in FIG. 40A in that the transistor 3200 is not provided. Also in this case, writing and retaining operation of data can be performed in a manner similar to the semiconductor device illustrated in FIG. 40A.

Next, reading of data of the semiconductor device illustrated in FIG. 40C is described. When the transistor 3300 is turned on, the third wiring 3003 which is in a floating state and the capacitor 3400 are electrically connected to each other, and the charge is redistributed between the third wiring 3003 and the capacitor 3400. As a result, the potential of the third wiring 3003 is changed. The amount of change in potential of the third wiring 3003 varies depending on the potential of the one electrode of the capacitor 3400 (or the charge accumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the charge redistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potential of the one electrode of the capacitor 3400, C is the capacitance of the capacitor 3400, C_(B) is the capacitance component of the third wiring 3003, and V_(B0) is the potential of the third wiring 3003 before the charge redistribution. Thus, it can be found that, assuming that the memory cell is in either of two states in which the potential of the one electrode of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potential of the third wiring 3003 in the case of retaining the potential V₁ (=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of the third wiring 3003 in the case of retaining the potential V₀ (=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with a predetermined potential, data can be read.

In this case, a transistor including the first semiconductor material may be used for a driver circuit for driving a memory cell, and a transistor including the second semiconductor material may be stacked over the driver circuit as the transistor 3300.

When including a transistor in which a channel formation region is formed using an oxide semiconductor and which has an extremely small off-state current, the semiconductor device described in this embodiment can retain stored data for an extremely long time. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be retained for a long time even when power is not supplied (note that a potential is preferably fixed).

Furthermore, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. Unlike in a conventional nonvolatile memory, for example, it is not necessary to inject and extract electrons into and from a floating gate; thus, a problem such as deterioration of a gate insulating film is not caused. That is, the semiconductor device of the disclosed invention does not have a limit on the number of times data can be rewritten, which is a problem of a conventional nonvolatile memory, and the reliability thereof is drastically improved. Furthermore, data is written depending on the state of the transistor (on or off), whereby high-speed operation can be easily achieved.

The memory device described in this embodiment can also be used in an LSI such as a central processing unit (CPU), a digital signal processor (DSP), a custom LSI, or a programmable logic device (PLD), and a radio frequency identification (RF-ID) tag, for example.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 8

In this embodiment, an RF device tag that includes the transistor or the memory device described in the above embodiment is described with reference to FIG. 41.

The RF device tag of this embodiment includes a memory circuit, stores necessary data in the memory circuit, and transmits and receives data to/from the outside by using contactless means, for example, wireless communication. With these features, the RF device tag can be used for an individual authentication system in which an object or the like is recognized by reading the individual information, for example. Note that the RF device tag is required to have extremely high reliability in order to be used for this purpose.

A configuration of the RF device tag will be described with reference to FIG. 41. FIG. 41 is a block diagram illustrating a configuration example of an RF device tag.

As shown in FIG. 41, an RF device tag 800 includes an antenna 804 which receives a radio signal 803 that is transmitted from an antenna 802 connected to a communication device 801 (also referred to as an interrogator, a reader/writer, or the like). The RF device tag 800 includes a rectifier circuit 805, a constant voltage circuit 806, a demodulation circuit 807, a modulation circuit 808, a logic circuit 809, a memory circuit 810, and a ROM 811. A transistor having a rectifying function included in the demodulation circuit 807 may be formed using a material which enables a reverse current to be low enough, for example, an oxide semiconductor. This can suppress the phenomenon of a rectifying function becoming weaker due to generation of a reverse current and prevent saturation of the output from the demodulation circuit. In other words, the input to the demodulation circuit and the output from the demodulation circuit can have a relation closer to a linear relation. Note that data transmission methods are roughly classified into the following three methods: an electromagnetic coupling method in which a pair of coils is provided so as to face each other and communicates with each other by mutual induction, an electromagnetic induction method in which communication is performed using an induction field, and a radio wave method in which communication is performed using a radio wave. Any of these methods can be used in the RF device tag 800 described in this embodiment.

Next, the structure of each circuit will be described. The antenna 804 exchanges the radio signal 803 with the antenna 802 which is connected to the communication device 801. The rectifier circuit 805 generates an input potential by rectification, for example, half-wave voltage doubler rectification of an input alternating signal generated by reception of a radio signal at the antenna 804 and smoothing of the rectified signal with a capacitor provided in a later stage in the rectifier circuit 805. Note that a limiter circuit may be provided on an input side or an output side of the rectifier circuit 805. The limiter circuit controls electric power so that electric power which is higher than or equal to certain electric power is not input to a circuit in a later stage if the amplitude of the input alternating signal is high and an internal generation voltage is high.

The constant voltage circuit 806 generates a stable power supply voltage from an input potential and supplies it to each circuit. Note that the constant voltage circuit 806 may include a reset signal generation circuit. The reset signal generation circuit is a circuit which generates a reset signal of the logic circuit 809 by utilizing rise of the stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal by envelope detection and generates the demodulated signal. Furthermore, the modulation circuit 808 performs modulation in accordance with data to be output from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. The memory circuit 810 holds the input data and includes a row decoder, a column decoder, a memory region, and the like. Furthermore, the ROM 811 stores an identification number (ID) or the like and outputs it in accordance with processing.

Note that the decision whether each circuit described above is provided or not can be made as appropriate as needed.

Here, the memory cell described in the above embodiment can be used as the memory circuit 810. Since the memory cell including an oxide semiconductor can retain data even when not powered, the memory circuit can be favorably used for an RF device tag. Furthermore, the memory cell including an oxide semiconductor needs power (voltage) for data writing significantly lower than that needed in a conventional nonvolatile memory; thus, it is possible to prevent a difference between the maximum communication range in data reading and that in data writing. In addition, it is possible to suppress malfunction or incorrect writing which is caused by power shortage in data writing.

Since the memory cell including an oxide semiconductor can be used as a nonvolatile memory, it can also be used as the ROM 811. In this case, it is preferable that a manufacturer separately prepare a command for writing data to the ROM 811 so that a user cannot rewrite data freely. Since the manufacturer gives identification numbers before shipment and then starts shipment of products, instead of putting identification numbers to all the manufactured RF device tags, it is possible to put identification numbers to only good products to be shipped. Thus, the identification numbers of the shipped products are in series and customer management for the shipped products is easily performed.

At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate.

Embodiment 9

In this embodiment, a CPU in which at least the transistor described in the above embodiment can be used and the memory device described in the above embodiment is included is described.

FIG. 42 is a block diagram illustrating a configuration example of a CPU at least partly including the transistor described in the above embodiment as a component.

The CPU illustrated in FIG. 42 includes, over a substrate 1190, an arithmetic logic unit (ALU) 1191, an ALU controller 1192, an instruction decoder 1193, an interrupt controller 1194, a timing controller 1195, a register 1196, a register controller 1197, a bus interface 1198 (BUS I/F), a rewritable ROM 1199, and a ROM interface (ROM I/F) 1189. A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate 1190. The ROM 1199 and the ROM interface 1189 may be provided over a separate chip. Needless to say, the CPU in FIG. 42 is just an example in which the configuration is simplified, and an actual CPU may have a variety of configurations depending on the application. For example, the CPU may have the following configuration: a structure including the CPU illustrated in FIG. 42 or an arithmetic circuit is considered as one core; a plurality of the cores are included; and the cores operate in parallel. The number of bits that the CPU can process in an internal arithmetic circuit or in a data bus can be, for example, 8, 16, 32, or 64.

An instruction that is input to the CPU through the bus interface 1198 is input to the instruction decoder 1193 and decoded therein, and then, input to the ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195 conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller 1192 generates signals for controlling the operation of the ALU 1191. While the CPU is executing a program, the interrupt controller 1194 processes an interrupt request from an external input/output device or a peripheral circuit depending on its priority or a mask state. The register controller 1197 generates an address of the register 1196, and reads/writes data from/to the register 1196 depending on the state of the CPU.

The timing controller 1195 generates signals for controlling operation timings of the ALU 1191, the ALU controller 1192, the instruction decoder 1193, the interrupt controller 1194, and the register controller 1197. For example, the timing controller 1195 includes an internal clock generator for generating an internal clock signal CLK2 on the basis of a reference clock signal CLK1, and supplies the internal clock signal CLK2 to the above circuits.

In the CPU illustrated in FIG. 42, a memory cell is provided in the register 1196. For the memory cell of the register 1196, any of the transistors described in the above embodiments can be used.

In the CPU illustrated in FIG. 42, the register controller 1197 selects operation of retaining data in the register 1196 in accordance with an instruction from the ALU 1191. That is, the register controller 1197 selects whether data is retained by a flip-flop or by a capacitor in the memory cell included in the register 1196. When data retaining by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register 1196. When data retaining by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register 1196 can be stopped.

FIG. 43 is an example of a circuit diagram of a memory element that can be used for the register 1196. A memory element 1200 includes a circuit 1201 in which stored data is volatile when power supply is stopped, a circuit 1202 in which stored data is nonvolatile even when power supply is stopped, a switch 1203, a switch 1204, a logic element 1206, a capacitor 1207, and a circuit 1220 having a selecting function. The circuit 1202 includes a capacitor 1208, a transistor 1209, and a transistor 1210. Note that the memory element 1200 may further include another element such as a diode, a resistor, or an inductor, as needed.

Here, the memory device described in the above embodiment can be used as the circuit 1202. When supply of a power supply voltage to the memory element 1200 is stopped, a ground potential (0 V) or a potential at which the transistor 1209 in the circuit 1202 is turned off continues to be input to a gate of the transistor 1209. For example, the gate of the transistor 1209 is grounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213 having one conductivity type (e.g., an n-channel transistor) and the switch 1204 is a transistor 1214 having a conductivity type opposite to the one conductivity type (e.g., a p-channel transistor). A first terminal of the switch 1203 corresponds to one of a source and a drain of the transistor 1213, a second terminal of the switch 1203 corresponds to the other of the source and the drain of the transistor 1213, and conduction or non-conduction between the first terminal and the second terminal of the switch 1203 (i.e., the on/off state of the transistor 1213) is selected by a control signal RD input to a gate of the transistor 1213. A first terminal of the switch 1204 corresponds to one of a source and a drain of the transistor 1214, a second terminal of the switch 1204 corresponds to the other of the source and the drain of the transistor 1214, and conduction or non-conduction between the first terminal and the second terminal of the switch 1204 (i.e., the on/off state of the transistor 1214) is selected by the control signal RD input to a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electrically connected to one of a pair of electrodes of the capacitor 1208 and a gate of the transistor 1210. Here, the connection portion is referred to as a node M2. One of a source and a drain of the transistor 1210 is electrically connected to a wiring that can supply a low power supply potential (e.g., a GND line), and the other thereof is electrically connected to the first terminal of the switch 1203 (the one of the source and the drain of the transistor 1213). The second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is electrically connected to the first terminal of the switch 1204 (the one of the source and the drain of the transistor 1214). The second terminal of the switch 1204 (the other of the source and the drain of the transistor 1214) is electrically connected to a wiring that can supply a power supply potential VDD. The second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213), the first terminal of the switch 1204 (the one of the source and the drain of the transistor 1214), an input terminal of the logic element 1206, and one of a pair of electrodes of the capacitor 1207 are electrically connected to each other. Here, the connection portion is referred to as a node M1. The other of the pair of electrodes of the capacitor 1207 can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor 1207 can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor 1207 is electrically connected to the wiring that can supply a low power supply potential (e.g., a GND line). The other of the pair of electrodes of the capacitor 1208 can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor 1208 can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor 1208 is electrically connected to the wiring that can supply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily provided as long as the parasitic capacitance of the transistor, the wiring, or the like is actively utilized.

A control signal WE is input to the first gate (first gate electrode) of the transistor 1209. As for each of the switch 1203 and the switch 1204, a conduction state or a non-conduction state between the first terminal and the second terminal is selected by the control signal RD that is different from the control signal WE. When the first terminal and the second terminal of one of the switches are in the conduction state, the first terminal and the second terminal of the other of the switches are in the non-conduction state.

A signal corresponding to data retained in the circuit 1201 is input to the other of the source and the drain of the transistor 1209. FIG. 43 illustrates an example in which a signal output from the circuit 1201 is input to the other of the source and the drain of the transistor 1209. The logic value of a signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is inverted by the logic element 1206, and the inverted signal is input to the circuit 1201 through the circuit 1220.

In the example of FIG. 43, a signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is input to the circuit 1201 through the logic element 1206 and the circuit 1220; however, one embodiment of the present invention is not limited thereto. The signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) may be input to the circuit 1201 without its logic value being inverted. For example, in the case where the circuit 1201 includes a node in which a signal obtained by inversion of the logic value of a signal input from the input terminal is retained, the signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) can be input to the node.

In FIG. 43, the transistors included in the memory element 1200 except for the transistor 1209 can each be a transistor in which a channel is formed in a layer formed using a semiconductor other than an oxide semiconductor or in the substrate 1190. For example, the transistor can be a transistor whose channel is formed in a silicon layer or a silicon substrate. Alternatively, a transistor in which a channel is formed in an oxide semiconductor film can be used for all the transistors in the memory element 1200. Further alternatively, in the memory element 1200, a transistor in which a channel is formed in an oxide semiconductor film can be included besides the transistor 1209, and a transistor in which a channel is formed in a layer formed using a semiconductor other than an oxide semiconductor or the substrate 1190 can be used for the rest of the transistors.

As the circuit 1201 in FIG. 43, for example, a flip-flop circuit can be used. As the logic element 1206, for example, an inverter or a clocked inverter can be used.

In a period during which the memory element 1200 is not supplied with the power supply voltage, the semiconductor device of one embodiment of the present invention can retain data stored in the circuit 1201 by the capacitor 1208 that is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in an oxide semiconductor film is extremely small. For example, the off-state current of a transistor in which a channel is formed in an oxide semiconductor film is significantly smaller than that of a transistor in which a channel is formed in silicon having crystallinity. Thus, when the transistor is used as the transistor 1209, a signal is retained in the capacitor 1208 for a long time also in a period during which the power supply voltage is not supplied to the memory element 1200. The memory element 1200 can accordingly retain the stored content (data) also in a period during which the supply of the power supply voltage is stopped.

Since the memory element performs pre-charge operation with the switch 1203 and the switch 1204, the time required for the circuit 1201 to retain original data again after the supply of the power supply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input to the gate of the transistor 1210. Thus, after supply of the power supply voltage to the memory element 1200 is restarted, the signal retained by the capacitor 1208 can be converted into the one corresponding to the state (the on state or the off state) of the transistor 1210 to be read from the circuit 1202. Consequently, an original signal can be accurately read even when a potential corresponding to the signal retained by the capacitor 1208 changes to some degree.

By using the above-described memory element 1200 in a memory device such as a register or a cache memory included in a processor, data in the memory device can be prevented from being lost owing to the stop of the supply of the power supply voltage. Furthermore, shortly after the supply of the power supply voltage is restarted, the memory device can be returned to the same state as that before the power supply is stopped. Thus, the power supply can be stopped even for a short time in the processor or one or a plurality of logic circuits included in the processor, resulting in lower power consumption.

Although the memory element 1200 is used in a CPU in this embodiment, the memory element 1200 can also be used in an LSI such as a digital signal processor (DSP), a custom LSI, or a programmable logic device (PLD), and a radio frequency identification (RF-ID).

At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate.

Embodiment 10 Imaging Device

An imaging device of one embodiment of the present invention is described below.

FIG. 44A is a plan view illustrating an example of an imaging device 2000 of one embodiment of the present invention. The imaging device 2000 includes a pixel portion 2010 and peripheral circuits for driving the pixel portion 2010 (a peripheral circuit 2060, a peripheral circuit 2070, a peripheral circuit 2080, and a peripheral circuit 2090). The pixel portion 2010 includes a plurality of pixels 2011 arranged in a matrix with p rows and q columns (p and q are each a natural number greater than or equal to 2). The peripheral circuit 2060, the peripheral circuit 2070, the peripheral circuit 2080, and the peripheral circuit 2090 are each connected to a plurality of pixels 2011, and a signal for driving the plurality of pixels 2011 is supplied. In this specification and the like, in some cases, “a peripheral circuit” or “a driver circuit” indicates all of the peripheral circuits 2060, 2070, 2080, and 2090. For example, the peripheral circuit 2060 can be regarded as part of the peripheral circuit.

The imaging device 2000 preferably includes a light source 2091. The light source 2091 can emit detection light P1.

The peripheral circuit includes at least one of a logic circuit, a switch, a buffer, an amplifier circuit, and a converter circuit. The peripheral circuit may be formed over a substrate where the pixel portion 2010 is formed. A semiconductor device such as an IC chip may be used as part or the whole of the peripheral circuit. Note that as the peripheral circuit, one or more of the peripheral circuits 2060, 2070, 2080, and 2090 may be omitted.

As illustrated in FIG. 44B, the pixels 2011 may be provided to be inclined in the pixel portion 2010 included in the imaging device 2000. When the pixels 2011 are obliquely arranged, the distance between pixels (pitch) can be shortened in the row direction and the column direction. Accordingly, the quality of an image taken with the imaging device 2000 can be improved.

<Configuration Example of Pixel>

The pixel 2011 included in the imaging device 2000 is formed with a plurality of subpixels 2012, and each subpixel 2012 is combined with a filter which transmits light with a specific wavelength band (color filter), whereby data for achieving color image display can be obtained.

FIG. 45A is a plan view showing an example of the pixel 2011 with which a color image is obtained. The pixel 2011 illustrated in FIG. 45A includes a subpixel 2012 provided with a color filter transmitting light with a red (R) wavelength band (also referred to as a subpixel 2012R), a subpixel 2012 provided with a color filter transmitting light with a green (G) wavelength band (also referred to as a subpixel 2012G), and a subpixel 2012 provided with a color filter transmitting light with a blue (B) wavelength band (also referred to as a subpixel 2012B). The subpixel 2012 can function as a photosensor.

The subpixel 2012 (the subpixel 2012R, the subpixel 2012G, and the subpixel 2012B) is electrically connected to a wiring 2031, a wiring 2047, a wiring 2048, a wiring 2049, and a wiring 2050. In addition, the subpixel 2012R, the subpixel 2012G, and the subpixel 2012B are connected to respective wirings 2053 which are independent from one another. In this specification and the like, for example, the wiring 2048 and the wiring 2049 that are connected to the pixel 2011 in the n-th row are referred to as a wiring 2048[n] and a wiring 2049[n]. For example, the wiring 2053 connected to the pixel 2011 in the m-th column is referred to as a wiring 2053[m]. Note that in FIG. 45A, the wirings 2053 connected to the subpixel 2012R, the subpixel 2012G, and the subpixel 2012B in the pixel 2011 in the m-th column are referred to as a wiring 2053[m]R, a wiring 2053[m]G, and a wiring 2053[m]B. The subpixels 2012 are electrically connected to the peripheral circuit through the above wirings.

The imaging device 2000 has a structure in which the subpixel 2012 is electrically connected to the subpixel 2012 in an adjacent pixel 2011 which is provided with a color filter transmitting light with the same wavelength band as the subpixel 2012, via a switch. FIG. 45B shows a connection example of the subpixels 2012: the subpixel 2012 in the pixel 2011 arranged in an n-th row (n is an integer greater than or equal to 1 and less than or equal to p) and an m-th column (m is an integer greater than or equal to 1 and less than or equal to q) and the subpixel 2012 in the adjacent pixel 2011 arranged in an (n+1)-th row and the m-th column. In FIG. 45B, the subpixel 2012R arranged in the n-th row and the m-th column and the subpixel 2012R arranged in the (n+1)-th row and the m-th column are connected to each other via a switch 2001. The subpixel 2012G arranged in the n-th row and the m-th column and the subpixel 2012G arranged in the (n+1)-th row and the m-th column are connected to each other via a switch 2002. The subpixel 2012B arranged in the n-th row and the m-th column and the subpixel 2012B arranged in the (n+1)-th row and the m-th column are connected to each other via a switch 2003.

The color filter used in the subpixel 2012 is not limited to red (R), green (G), and blue (B) color filters, and color filters that transmit light of cyan (C), yellow (Y), and magenta (M) may be used. By provision of the subpixels 2012 that sense light with three different wavelength bands in one pixel 2011, a full-color image can be obtained.

The pixel 2011 including the subpixel 2012 provided with a color filter transmitting yellow (Y) light may be provided, in addition to the subpixels 2012 provided with the color filters transmitting red (R), green (G), and blue (B) light. The pixel 2011 including the subpixel 2012 provided with a color filter transmitting blue (B) light may be provided, in addition to the subpixels 2012 provided with the color filters transmitting cyan (C), yellow (Y), and magenta (M) light. When the subpixels 2012 sensing light with four different wavelength bands are provided in one pixel 2011, the reproducibility of colors of an obtained image can be increased.

For example, in FIG. 45A, in regard to the subpixel 2012 sensing a red wavelength band, the subpixel 2012 sensing a green wavelength band, and the subpixel 2012 sensing a blue wavelength band, the pixel number ratio (or the light receiving area ratio) thereof is not necessarily 1:1:1. For example, the Bayer arrangement in which the pixel number ratio (the light receiving area ratio) is set at red:green:blue=1:2:1 may be employed. Alternatively, the pixel number ratio (the light receiving area ratio) of red and green to blue may be 1:6:1.

Although the number of subpixels 2012 provided in the pixel 2011 may be one, two or more subpixels are preferably provided. For example, when two or more subpixels 2012 sensing the same wavelength band are provided, the redundancy is increased, and the reliability of the imaging device 2000 can be increased.

When an infrared (IR) filter that transmits infrared light and absorbs or reflects visible light is used, the imaging device 2000 that senses infrared light can be achieved.

Furthermore, when a neutral density (ND) filter (dark filter) is used, output saturation which occurs when a large amount of light enters a photoelectric conversion element (light-receiving element) can be prevented. With a combination of ND filters with different dimming capabilities, the dynamic range of the imaging device can be increased.

Besides the above-described filter, the pixel 2011 may be provided with a lens. An arrangement example of the pixel 2011, a filter 2054, and a lens 2055 is described with cross-sectional views in FIGS. 46A and 46B. With the lens 2055, the photoelectric conversion element can receive incident light efficiently. Specifically, as illustrated in FIG. 46A, light 2056 enters a photoelectric conversion element 2020 through the lens 2055, the filter 2054 (a filter 2054R, a filter 2054G, and a filter 2054B), a pixel circuit 2030, and the like which are provided in the pixel 2011.

However, as indicated by a region surrounded with dashed-dotted lines, part of the light 2056 indicated by arrows might be blocked by some wirings 2057. Thus, a preferable structure is that the lens 2055 and the filter 2054 are provided on the photoelectric conversion element 2020 side, so that the photoelectric conversion element 2020 can efficiently receive the light 2056 as illustrated in FIG. 46B. When the light 2056 enters the photoelectric conversion element 2020 from the photoelectric conversion element 2020 side, the imaging device 2000 with high sensitivity can be provided.

As the photoelectric conversion element 2020 illustrated in FIGS. 46A and 46B, a photoelectric conversion element in which a p-n junction or a p-i-n junction is formed may be used.

The photoelectric conversion element 2020 may be formed using a substance that has a function of absorbing a radiation and generating electric charges. Examples of the substance that has a function of absorbing a radiation and generating electric charges include selenium, lead iodide, mercury iodide, gallium arsenide, cadmium telluride, and cadmium zinc alloy.

For example, when selenium is used for the photoelectric conversion element 2020, the photoelectric conversion element 2020 can have a light absorption coefficient in a wide wavelength range, such as visible light, ultraviolet light, infrared light, X-rays, and gamma rays.

One pixel 2011 included in the imaging device 2000 may include the subpixel 2012 with a first filter in addition to the subpixel 2012 illustrated in FIGS. 45A and 45B.

<Structure Example of Imaging Device>

FIG. 47 is a cross-sectional view of an element included in an imaging device. The imaging device illustrated in FIG. 47 includes the layer 621, the layer 620 over the layer 621, and the layer 622 over the layer 620. The layer 621 includes the transistor 491 provided over the substrate 400 and a photodiode 2360 provided over the substrate 400. The layer 622 includes the transistor 490 and a transistor 490 b. Note that any of the transistors described in Embodiment 1 can be used as the transistors 490 and 490 b.

The photodiode 2360 included in the imaging device includes an electrode 2361, an electrode 2362, and a semiconductor layer sandwiched between the two electrodes. The electrode 2362 is connected to the conductive layer 343 b included in the layer 620 through a conductive layer 312 b or the like included in the layer 621. The conductive layer 313 b is in contact with at least part of top and side surfaces of the projection portion of the conductive layer 343 b. The conductive layer 313 b is connected to the conductive layer 416 b included in the transistor 490 through a conductive layer 344 b or the like included in the layer 622. Part of the conductive layer 313 b is preferably in contact with the top surface of the insulating film 371.

The electrode 2361 is connected to a conductive layer 341 c provided over a low-resistance layer 2363 through the low-resistance layer 2363 and also connected to a conductive layer 343 e included in the layer 620 through other wiring layers such as a conductive layer 312 c included in the layer 621. The conductive layer 313 e is in contact with at least part of top and side surfaces of a projection portion of the conductive layer 343 e. The conductive layer 313 e is connected to, for example, other wiring layers included in the layer 622 through a conductive layer 344 c or the like included in the layer 622. Part of the conductive layer 313 e is preferably in contact with the top surface of the insulating film 371.

For the conductive layer 313 b and 313 e, the description of the conductive layer 313 can be referred to. For the conductive layers 343 b and 343 e, the description of the conductive layer 343 can be referred to.

In the example of cross-sectional view in FIG. 47, a light-receiving surface of the photodiode 2360 is positioned on the side opposite to a surface of the substrate 400 where the transistor 491 is provided. This structure can ensure the light path without the influences of various transistors, wirings, and the like. Thus, a pixel with a high aperture ratio can be formed. Note that the light-receiving surface of the photodiode 2360 can be positioned on the same side as the surface where the transistor 491 is formed.

At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate.

Example

In this example, transistors corresponding to the transistor 100 illustrated in FIGS. 1A to 1C and the transistor 171 illustrated in FIGS. 8A to 8C were fabricated, and their Id-Vg characteristics were evaluated.

Sample A1 and Sample A2 described below were fabricated and used for evaluation in this example. Samples A1 and A2 were samples of embodiments of the present invention. Sample A1 is a transistor including one gate electrode, and Sample A2 is a transistor including two gate electrodes.

Structures of the transistors in this example are described with reference to FIGS. 37A and 37B. FIG. 37A is a top view of the transistor 100 and FIG. 37B is a top view of the transistor 171. In each of Samples A1 and A2, a length 123 of a gate electrode was 6 μm. A distance 122 a (also referred to as an offset length of a source in this specification) between the conductive film 112 a functioning as a source electrode and the conductive film 104 functioning as a first gate electrode and a distance 122 b (also referred to as an offset length of a drain in this specification) between the conductive film 112 b functioning as a drain electrode and the conductive film 104 functioning as the first gate electrode were each 1.5 μm. A width 124 of the oxide semiconductor film 108 was 10 μm.

Samples A1 and A2 fabricated in this example are described below. Note that the reference numerals used for the transistor 100 in FIGS. 1A to 1C and the transistor 171 in FIGS. 8A to 8C are used in the following description. Note that there were steps which were subjected to Sample A2, and not to Sample A1 in the fabrication process.

<Fabrication Method of Sample A1 and Sample A2>

First, the conductive film 104 was formed over the substrate 102. As the substrate 102, a glass substrate was used. Note that the size and thickness of the glass substrate were 600 mm×720 mm and 0.7 mm, respectively. Furthermore, as the conductive film 104, a 100-nm-thick tungsten film was formed with a sputtering apparatus.

Next, the insulating films 106 and 107 were formed over the substrate 102 and the conductive film 104. As the insulating film 106, a 400-nm-thick silicon nitride film was formed with a PECVD apparatus. As the insulating film 107, a 50-nm-thick silicon oxynitride film was formed with a PECVD apparatus.

The insulating film 106 was deposited as follows. First, a 50-nm-thick silicon nitride film was deposited under the conditions where the substrate temperature was 350° C.; a silane gas at a flow rate of 200 sccm, a nitrogen gas at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm were introduced into a chamber; the pressure was set to 100 Pa; and an RF power of 2000 W was supplied between parallel-plate electrodes placed in a PECVD apparatus. Then, the flow rate of an ammonia gas was changed to 2000 sccm to deposit a 300-nm-thick silicon nitride film. Finally, the flow rate of an ammonia gas was changed to 100 sccm to deposit a 50-nm-thick silicon nitride film.

The insulating film 107 was deposited under the conditions where the substrate temperature was 350° C.; a silane gas at a flow rate of 20 sccm and a dinitrogen monoxide gas at a flow rate of 3000 sccm were introduced into a chamber; the pressure was set to 40 Pa; and an RF power of 100 W was supplied between parallel-plate electrodes provided in a PECVD apparatus.

Then, the oxide semiconductor film 108 was formed over the insulating film 107. As the oxide semiconductor film 108, a stacked layer in which the first oxide semiconductor film 108 a on the conductive film 104 side and the second oxide semiconductor film 108 b over the first oxide semiconductor film 108 a was formed. The conductive film 104 serves as a first gate electrode. A 10-nm-thick IGZO film and a 15-nm-thick IGZO film were formed as the first oxide semiconductor film 108 a and the second oxide semiconductor film 108 b, respectively.

Note that the first oxide semiconductor film 108 a was deposited under the conditions where the substrate temperature was 170° C.; an argon gas at a flow rate of 140 sccm and an oxygen gas at a flow rate of 60 sccm were introduced into a chamber; the pressure was 0.6 Pa; and an AC power of 2500 W was applied to a polycrystalline metal oxide sputtering target (having an atomic ratio of In:Ga:Zn=4:2:4.1).

Note that the second oxide semiconductor film 108 b was deposited under the conditions where the substrate temperature was 170° C.; an argon gas at a flow rate of 100 sccm and an oxygen gas at a flow rate of 100 sccm were introduced into a chamber; the pressure was 0.6 Pa; and an AC power of 2500 W was applied to a polycrystalline metal oxide sputtering target (having an atomic ratio of In:Ga:Zn=1:1:1.2).

Then, first heat treatment was performed. As the first heat treatment, heat treatment was performed at 450° C. in a nitrogen gas atmosphere for 1 hour and then heat treatment was performed at 450° C. in a mixed atmosphere of a nitrogen gas and an oxygen gas for 1 hour.

Next, the conductive films 112 a and 112 b were formed over the insulating film 107 and the oxide semiconductor film 108. As the conductive films 112 a and 112 b, a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film were successively formed in a vacuum with a sputtering apparatus.

After that, the insulating film 114 and the insulating film 116 were formed over the insulating film 107, the oxide semiconductor film 108, and the conductive films 112 a and 112 b. As the insulating film 114, a 50-nm-thick silicon oxynitride film was formed with a PECVD apparatus. As the insulating film 116, a 400-nm-thick silicon oxynitride film was formed with a PECVD apparatus. Note that the insulating film 114 and the insulating film 116 were successively formed in vacuum with a PECVD apparatus.

The insulating film 114 was deposited under the conditions where the substrate temperature was 220° C.; a silane gas at a flow rate of 50 sccm and a dinitrogen monoxide gas at a flow rate of 2000 sccm were introduced into a chamber; the pressure was 20 Pa; and an RF power of 100 W was supplied between parallel-plate electrodes provided in a PECVD apparatus. The insulating film 116 was deposited under the conditions where the substrate temperature was 220° C.; a silane gas at a flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of 4000 sccm were introduced into a chamber; the pressure was 200 Pa; and an RF power of 1500 W was supplied between parallel-plate electrodes provided in a PECVD apparatus.

Then, second heat treatment was performed. The second heat treatment was performed at 350° C. in a nitrogen gas atmosphere for 1 hour.

A 5-nm-thick ITSO film was formed over the insulating film 116 with a sputtering apparatus. The ITSO film was deposited under the conditions where the substrate temperature was room temperature; an argon gas at a flow rate of 72 sccm and an oxygen gas at a flow rate of 5 sccm were introduced into a chamber; the pressure was 0.15 Pa; and a DC power of 1000 W was supplied to a metal oxide target (In₂O₃:SnO₂:SiO₂=85:10:5 [wt. %]) provided in a sputtering apparatus.

Next, oxygen addition treatment was performed on the insulating film 116 through the ITSO film. The oxygen addition treatment was performed with an ashing apparatus under the conditions where the substrate temperature was 40° C.; an oxygen gas at a flow rate of 250 sccm was introduced into a chamber; the pressure was 15 Pa; and an RF power of 4500 W was supplied for 120 sec. between parallel-plate electrodes provided in the ashing apparatus so that a bias would be applied to the substrate side.

Then, the ITSO film was removed to expose the insulating film 116. The ITSO film was removed with a wet-etching apparatus in such a manner that etching was performed using an oxalic acid solution at a concentration of 5% for 300 sec. and then etching was performed using hydrofluoric acid at a concentration of 0.5% for 15 sec.

Next, the insulating film 118 was formed over the insulating film 116. As the insulating film 118, a 100-nm-thick silicon nitride film was formed with a PECVD apparatus. The insulating film 118 was deposited under the conditions where the substrate temperature was 350° C.; a silane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of 5000 sccm, and an ammonia gas at a flow rate of 100 sccm were introduced into a chamber; the pressure was 100 Pa; and an RF power of 1000 W was supplied between parallel-plate electrodes provided in a PECVD apparatus.

Next, only in Sample A2, the opening 142 c reaching the conductive film 112 b and the openings 142 a and 142 b reaching the conductive film 104 were formed. The openings 142 a, 142 b, and 142 c were formed with a dry etching apparatus.

Next, in only Sample A2, a conductive film was formed over the insulating film 118 to cover the openings 142 a, 142 b, and 142 c and processed to form the conductive films 120 a and 120 b. For the conductive films 120 a and 120 b, a 100-nm-thick ITSO film was formed with a sputtering apparatus. The composition of a target used for forming the ITSO film was the same as that used for forming the ITSO film described above.

Next, a 1.5-μm-thick photosensitive acrylic film was formed over the insulating film 118 in Sample A1 and over the insulating film 118, the conductive film 120 a, and the conductive film 120 b in Sample A2 by a spin coating method and then light exposure was performed.

Then, third heat treatment was performed. The third heat treatment was performed at 250° C. in a nitrogen gas atmosphere for 1 hour.

Then, fourth heat treatment was performed. The fourth heat treatment was performed at 250° C. in a nitrogen gas atmosphere for 1 hour.

Through the above process, Samples A1 and A2 of this example were fabricated.

<Id-Vg Characteristics of Transistors>

Next, Id-Vg characteristics of Samples A1 and A2 were measured. FIGS. 38A and 38B show Id-Vg characteristics of Samples A1 and A2. In FIGS. 38A and 38B, the first vertical axis indicates Id (A), the second vertical axis indicates μFE (cm²/Vs), and the horizontal axis indicates Vg (V).

In measuring the Id-Vg characteristics of the transistor in Sample A1, a voltage (hereinafter also referred to as gate voltage (Vg)) applied to the conductive film 104 that functions as the first gate electrode was changed from −15 V to +15 V in increments of 0.25 V. In measuring the Id-Vg characteristics of the transistor in Sample A2, a voltage (Vg) applied to the conductive film 104 that functions as the first gate electrode and a voltage (Vbg) applied to the conductive film 120 b that functions as a second gate electrode were each changed from −15 V to +15 V in increments of 0.25 V. In Samples A1 and A2, a voltage (hereinafter also referred to as source voltage (Vs)) applied to the conductive film 112 a that functions as a source electrode is 0 V (comm), and a voltage (hereinafter also referred to as drain voltage (Vd)) applied to the conductive film 112 b that functions as a drain electrode is 0.1 V or 10 V. Note that the results of field-effect mobility (μFE) when Vd=10 V are shown.

The field-effect mobility was obtained from the formula of saturation mobility using the length 123 of the gate electrode as a channel length L (L=6 μm) and the width 124 of the oxide semiconductor film 108 as a channel width W (W=10 μm) in FIGS. 37A and 37B. Note that here, field-effect mobility in a saturation region is described as the saturation mobility. The maximum value of the saturation mobility obtained by calculation is an index of current drive capability in the saturation region (gate voltage V_(g)<drain voltage V_(d)+ threshold voltage V_(th)), and is not an approximate value of the mobility as the physical property of the oxide semiconductor film. The capacitance of an insulating film was calculated using the capacitance of the insulating films 106 and 107 functioning as first gate insulating films also in the case of Sample A2.

It was found from the results in FIG. 38A that the maximum field-effect mobility was 2 cm²/Vs or more when the offset length of each of the source and the drain was 1.5 μm in the transistor in Sample A1. Furthermore, it was found from the results in FIG. 38B that the maximum field-effect mobility was 5 cm²/Vs or more when the offset length of each of the source and the drain was 1.5 μm in the transistor in Sample A2.

The above results show that excellent electrical characteristics such as high field-effect mobility can be obtained by use of a stacked structure of oxide semiconductor films in a transistor having a structure where a first gate electrode does not overlap with a source electrode and a drain electrode in the semiconductor device of one embodiment of the present invention.

The structure described above in this example can be combined with any of the structures described in the embodiments as appropriate.

This application is based on Japanese Patent Application serial no. 2015-025021 filed with Japan Patent Office on Feb. 12, 2015, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a transistor comprising: an oxide semiconductor film; a first gate electrode, the oxide semiconductor film overlapping with the first gate electrode; a first gate insulating film between the oxide semiconductor film and the first gate electrode; a source electrode electrically connected to the oxide semiconductor film; and a drain electrode electrically connected to the oxide semiconductor film, wherein the oxide semiconductor film comprises: a first oxide semiconductor film in contact with the first gate insulating film; and a second oxide semiconductor film in contact with the first oxide semiconductor film, wherein an atomic proportion of In in the first oxide semiconductor film is larger than an atomic proportion of M in the first oxide semiconductor film, wherein M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf, wherein an atomic proportion of In in the second oxide semiconductor film is smaller than the atomic proportion of In in the first oxide semiconductor film, wherein the oxide semiconductor film comprises a first region overlapping with the first gate electrode, a second region overlapping with the source electrode, and a third region overlapping with the drain electrode, wherein the first region does not comprise the second region and the third region, wherein the first oxide semiconductor film has a first thickness, wherein the second oxide semiconductor film has a second thickness overlapped with the first gate electrode and a third thickness overlapped with the source electrode and the drain electrode, wherein the third thickness is larger than the first thickness, and wherein the first thickness is larger than the second thickness.
 2. The semiconductor device according to claim 1, wherein the first gate insulating film is over the first gate electrode, and wherein the oxide semiconductor film is over the first gate insulating film.
 3. The semiconductor device according to claim 2, further comprising: a second gate insulating film over the oxide semiconductor film; and a second gate electrode over the second gate insulating film, wherein the oxide semiconductor film further comprises a fourth region overlapping with the second gate electrode, and wherein the fourth region does not comprise the second region and the third region.
 4. The semiconductor device according to claim 1, wherein the first gate insulating film is over the oxide semiconductor film, and wherein the first gate electrode is over the first gate insulating film.
 5. The semiconductor device according to claim 4, further comprising: a second gate insulating film under the oxide semiconductor film; and a second gate electrode under the second gate insulating film, wherein the oxide semiconductor film further comprises a fourth region overlapping with the second gate electrode, and wherein the fourth region does not comprise the second region and the third region.
 6. The semiconductor device according to claim 1, further comprising: a scan line; and a data line, wherein the first gate electrode is electrically connected to the scan line, wherein the source electrode is electrically connected to the data line, and wherein an insulating film having a larger thickness than the first gate insulating film is between the scan line and the data line in a region where the scan line and the data line overlap with each other with the insulating film therebetween.
 7. The semiconductor device according to claim 1, wherein the first oxide semiconductor film comprises In, M, and Zn, and wherein M is Ga.
 8. The semiconductor device according to claim 1, wherein each of the first oxide semiconductor film and the second oxide semiconductor film comprise a crystal portion, and wherein the crystal portion comprises a portion where a c-axis of the crystal portion is parallel to a normal vector of a surface where the oxide semiconductor film is formed.
 9. The semiconductor device according to claim 8, wherein the first oxide semiconductor film comprises a portion in which a proportion of the crystal portion is larger than a proportion of the crystal portion in the second oxide semiconductor film.
 10. The semiconductor device according to claim 1, wherein the first oxide semiconductor film comprises a portion in which a hydrogen concentration is lower than a hydrogen concentration in the second oxide semiconductor film.
 11. An electronic device comprising: the semiconductor device according to claim 1; and an operation key or a battery.
 12. A semiconductor device comprising: a transistor comprising: a first gate electrode; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film; a source electrode electrically connected to the oxide semiconductor film; a drain electrode electrically connected to the oxide semiconductor film; a second gate insulating film over the oxide semiconductor film; and a second gate electrode over the second gate insulating film, wherein the oxide semiconductor film comprises: a first oxide semiconductor film; and a second oxide semiconductor film over the first oxide semiconductor film, wherein an atomic proportion of In in the first oxide semiconductor film is larger than an atomic proportion of M in the first oxide semiconductor film, wherein M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf, wherein an atomic proportion of In in the second oxide semiconductor film is smaller than the atomic proportion of In in the first oxide semiconductor film, wherein the oxide semiconductor film comprises a first region overlapping with the first gate electrode, a second region overlapping with the source electrode, a third region overlapping with the drain electrode, and a fourth region overlapping with the second gate electrode, wherein the first region or the fourth region does not comprise the second region and the third region, wherein the first oxide semiconductor film has a first thickness, wherein the second oxide semiconductor film has a second thickness overlapped with the first gate electrode and a third thickness overlapped with the source electrode and the drain electrode, wherein the third thickness is larger than the first thickness, and wherein the first thickness is larger than the second thickness.
 13. The semiconductor device according to claim 12, further comprising: a scan line; and a data line, wherein the first gate electrode is electrically connected to the scan line, wherein the source electrode is electrically connected to the data line, and wherein an insulating film having a larger thickness than the first gate insulating film is between the scan line and the data line in a region where the scan line and the data line overlap with each other with the insulating film therebetween.
 14. The semiconductor device according to claim 12, wherein the first oxide semiconductor film comprises In, M, and Zn, and wherein M is Ga.
 15. The semiconductor device according to claim 12, wherein each of the first oxide semiconductor film and the second oxide semiconductor film comprises a crystal portion, and wherein the crystal portion comprises a portion where a c-axis of the crystal portion is parallel to a normal vector of a surface where the oxide semiconductor film is formed.
 16. The semiconductor device according to claim 15, wherein the first oxide semiconductor film comprises a portion in which a proportion of the crystal portion is larger than a proportion of the crystal portion in the second oxide semiconductor film.
 17. The semiconductor device according to claim 12, wherein the first oxide semiconductor film comprises a portion in which a hydrogen concentration is lower than a hydrogen concentration in the second oxide semiconductor film.
 18. An electronic device comprising: the semiconductor device according to claim 12; and an operation key or a battery.
 19. A semiconductor device comprising: a transistor comprising: a first gate electrode; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film; a source electrode electrically connected to the oxide semiconductor film; a drain electrode electrically connected to the oxide semiconductor film; a second gate insulating film over the oxide semiconductor film; and a second gate electrode over the second gate insulating film, wherein the oxide semiconductor film comprises: a first oxide semiconductor film; and a second oxide semiconductor film over the first oxide semiconductor film, wherein an atomic proportion of In in the first oxide semiconductor film is larger than an atomic proportion of M in the first oxide semiconductor film, wherein M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or I-If, wherein an atomic proportion of In in the second oxide semiconductor film is smaller than the atomic proportion of In in the first oxide semiconductor film, wherein the oxide semiconductor film comprises a first region overlapping with the first gate electrode, a second region overlapping with the source electrode, a third region overlapping with the drain electrode, and a fourth region overlapping with the second gate electrode, wherein the first region or the fourth region does not comprise the second region or the third region, wherein the first oxide semiconductor film has a first thickness, wherein the second oxide semiconductor film has a second thickness overlapped with the first gate electrode and a third thickness overlapped with the source electrode and the drain electrode, wherein the third thickness is larger than the first thickness, and wherein the first thickness is larger than the second thickness.
 20. The semiconductor device according to claim 19, further comprising: a scan line; and a data line, wherein the first gate electrode is electrically connected to the scan line, wherein the source electrode is electrically connected to the data line, and wherein an insulating film having a larger thickness than the first gate insulating film is between the scan line and the data line in a region where the scan line and the data line overlap with each other with the insulating film therebetween.
 21. The semiconductor device according to claim 19, wherein the first oxide semiconductor film comprises In, M, and Zn, and wherein M is Ga.
 22. The semiconductor device according to claim 19, wherein each of the first oxide semiconductor film and the second oxide semiconductor film comprises a crystal portion, and wherein the crystal portion comprises a portion where a c-axis of the crystal portion is parallel to a normal vector of a surface where the oxide semiconductor film is formed.
 23. The semiconductor device according to claim 22, wherein the first oxide semiconductor film comprises a portion in which a proportion of the crystal portion is larger than a proportion of the crystal portion in the second oxide semiconductor film.
 24. The semiconductor device according to claim 19, wherein the first oxide semiconductor film comprises a portion in which a hydrogen concentration is lower than a hydrogen concentration in the second oxide semiconductor film.
 25. An electronic device comprising: the semiconductor device according to claim 19; and an operation key or a battery.
 26. A display device comprising: an oxide semiconductor film; a first gate electrode, the oxide semiconductor film overlapping with the first gate electrode; a first gate insulating film between the oxide semiconductor film and the first gate electrode; a source electrode electrically connected to the oxide semiconductor film; and a drain electrode electrically connected to the oxide semiconductor film, wherein the oxide semiconductor film comprises: a first oxide semiconductor film in contact with the first gate insulating film; a second oxide semiconductor film in contact with the first oxide semiconductor film; and a conductive film electrically connected to one of the source electrode and the drain electrode, wherein an atomic proportion of In in the first oxide semiconductor film is larger than an atomic proportion of M in the first oxide semiconductor film, wherein M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf, wherein the oxide semiconductor film comprises a first region overlapping with the first gate electrode, a second region overlapping with the source electrode, and a third region overlapping with the drain electrode, wherein the first oxide semiconductor film has a first thickness, wherein the second oxide semiconductor film has a second thickness overlapped with the first gate electrode and a third thickness overlapped with the source electrode and the drain electrode, wherein the third thickness is larger than the first thickness, and wherein the first thickness is larger than the second thickness.
 27. The display device according to claim 26, further comprising an insulating film over the oxide semiconductor film, the source electrode, and the drain electrode, wherein the conductive film is over the insulating film, and wherein the conductive film is in contact with the one of the source electrode and the drain electrode through an opening provided in the insulating film.
 28. The display device according to claim 26, further comprising a liquid crystal layer over the conductive film.
 29. The display device according to claim 26, wherein an atomic proportion of In in the second oxide semiconductor film is smaller than the atomic proportion of in in the first oxide semiconductor film.
 30. The display device according to claim 26, wherein the first oxide semiconductor film comprises In, M, and Zn, and wherein M is Ga. 